Added nv-windows-dangling-transform-feedback-varying-names workaround.

On Windows NVidia drivers the glTransformFeedbackVaryings() does not
make a copy of its char* arguments so it fails at link time when the
original char arrays are not in scope anymore. Enabling *synchronous*
debug output circumvents this bug. Can be triggered by running
TransformFeedbackGLTest with GL_KHR_debug extension disabled.

src/Magnum/AbstractShaderProgram.cpp

@@ -368,6 +368,10 @@ void AbstractShaderProgram::bindFragmentDataLocationIndexedInternal(const Unsign
 
 #ifndef MAGNUM_TARGET_GLES2
 void AbstractShaderProgram::setTransformFeedbackOutputs(const std::initializer_list<std::string> outputs, const TransformFeedbackBufferMode bufferMode) {
+    (this->*Context::current().state().shaderProgram->transformFeedbackVaryingsImplementation)({outputs.begin(), outputs.size()}, bufferMode);
+}
+
+void AbstractShaderProgram::transformFeedbackVaryingsImplementationDefault(const Containers::ArrayView<const std::string> outputs, const TransformFeedbackBufferMode bufferMode) {
     /** @todo VLAs */
     Containers::Array<const char*> names{outputs.size()};
 
@@ -376,6 +380,19 @@ void AbstractShaderProgram::setTransformFeedbackOutputs(const std::initializer_l
 
     glTransformFeedbackVaryings(_id, outputs.size(), names, GLenum(bufferMode));
 }
+
+#ifdef CORRADE_TARGET_WINDOWS
+void AbstractShaderProgram::transformFeedbackVaryingsImplementationDanglingWorkaround(const Containers::ArrayView<const std::string> outputs, const TransformFeedbackBufferMode bufferMode) {
+    /* NVidia on Windows doesn't copy the names when calling
+       glTransformFeedbackVaryings() so it then fails at link time because the
+       char* are dangling. We have to do the copy on the engine side and keep
+       the values until link time (which can happen any time and multiple
+       times, so basically for the remaining lifetime of the shader program) */
+    _transformFeedbackVaryingNames.assign(outputs.begin(), outputs.end());
+
+    transformFeedbackVaryingsImplementationDefault({_transformFeedbackVaryingNames.data(), _transformFeedbackVaryingNames.size()}, bufferMode);
+}
+#endif
 #endif
 
 bool AbstractShaderProgram::link(std::initializer_list<std::reference_wrapper<AbstractShaderProgram>> shaders) {

src/Magnum/AbstractShaderProgram.h

@@ -36,6 +36,10 @@
 #include "Magnum/AbstractObject.h"
 #include "Magnum/Attribute.h"
 
+#if defined(CORRADE_TARGET_WINDOWS) && !defined(MAGNUM_TARGET_GLES2)
+#include <vector>
+#endif
+
 namespace Magnum {
 
 namespace Implementation { struct ShaderProgramState; }
@@ -1189,6 +1193,13 @@ class MAGNUM_EXPORT AbstractShaderProgram: public AbstractObject {
         Int uniformLocationInternal(Containers::ArrayView<const char> name);
         UnsignedInt uniformBlockIndexInternal(Containers::ArrayView<const char> name);
 
+        #ifndef MAGNUM_TARGET_GLES2
+        void MAGNUM_LOCAL transformFeedbackVaryingsImplementationDefault(Containers::ArrayView<const std::string> outputs, TransformFeedbackBufferMode bufferMode);
+        #ifdef CORRADE_TARGET_WINDOWS
+        void MAGNUM_LOCAL transformFeedbackVaryingsImplementationDanglingWorkaround(Containers::ArrayView<const std::string> outputs, TransformFeedbackBufferMode bufferMode);
+        #endif
+        #endif
+
         #ifndef MAGNUM_BUILD_DEPRECATED
         void use();
         #endif
@@ -1347,6 +1358,12 @@ class MAGNUM_EXPORT AbstractShaderProgram: public AbstractObject {
         #endif
 
         GLuint _id;
+
+        #if defined(CORRADE_TARGET_WINDOWS) && !defined(MAGNUM_TARGET_GLES2)
+        /* Needed for the nv-windows-dangling-transform-feedback-varying-names
+           workaround */
+        std::vector<std::string> _transformFeedbackVaryingNames;
+        #endif
 };
 
 }

src/Magnum/Implementation/ShaderProgramState.cpp

@@ -47,6 +47,19 @@ ShaderProgramState::ShaderProgramState(Context& context, std::vector<std::string
         , maxImageSamples(0)
         #endif
 {
+    #ifndef MAGNUM_TARGET_GLES2
+    #ifdef CORRADE_TARGET_WINDOWS
+    if((context.detectedDriver() & Context::DetectedDriver::NVidia) &&
+        !context.isDriverWorkaroundDisabled("nv-windows-dangling-transform-feedback-varying-names"))
+    {
+        transformFeedbackVaryingsImplementation = &AbstractShaderProgram::transformFeedbackVaryingsImplementationDanglingWorkaround;
+    } else
+    #endif
+    {
+        transformFeedbackVaryingsImplementation = &AbstractShaderProgram::transformFeedbackVaryingsImplementationDefault;
+    }
+    #endif
+
     #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
     #ifndef MAGNUM_TARGET_GLES
     if(context.isExtensionSupported<Extensions::GL::ARB::separate_shader_objects>())

src/Magnum/Implementation/ShaderProgramState.h

@@ -32,10 +32,7 @@
 #include "Magnum/OpenGL.h"
 #include "Magnum/Math/Vector3.h"
 
-#ifdef _MSC_VER
+#ifndef MAGNUM_TARGET_GLES2
-/* Otherwise the member function pointers will have different size based on
-   whether the header was included or not. CAUSES SERIOUS MEMORY CORRUPTION AND
-   IS NOT CAUGHT BY ANY WARNING WHATSOEVER! AARGH! */
 #include "Magnum/AbstractShaderProgram.h"
 #endif
 
@@ -46,6 +43,10 @@ struct ShaderProgramState {
 
     void reset();
 
+    #ifndef MAGNUM_TARGET_GLES2
+    void(AbstractShaderProgram::*transformFeedbackVaryingsImplementation)(Containers::ArrayView<const std::string>, AbstractShaderProgram::TransformFeedbackBufferMode);
+    #endif
+
     void(AbstractShaderProgram::*uniform1fvImplementation)(GLint, GLsizei, const GLfloat*);
     void(AbstractShaderProgram::*uniform2fvImplementation)(GLint, GLsizei, const Math::Vector<2, GLfloat>*);
     void(AbstractShaderProgram::*uniform3fvImplementation)(GLint, GLsizei, const Math::Vector<3, GLfloat>*);

src/Magnum/Implementation/driverSpecific.cpp

@@ -48,6 +48,14 @@ namespace {
            GLSL even though the extension (e.g. binding keyword) is not
            supported */
         "intel-windows-glsl-exposes-unsupported-shading-language-420pack",
+
+        /* On Windows NVidia drivers the glTransformFeedbackVaryings() does not
+           make a copy of its char* arguments so it fails at link time when the
+           original char arrays are not in scope anymore. Enabling
+           *synchronous* debug output circumvents this bug. Can be triggered by
+           running TransformFeedbackGLTest with GL_KHR_debug extension
+           disabled. */
+        "nv-windows-dangling-transform-feedback-varying-names",
         #endif
 
         /* Layout qualifier causes compiler error with GLSL 1.20 on Mesa, GLSL