GL: add a NoCreate Shader constructor.

It wasn't there because I thought it was not needed. I need it now.

doc/changelog.dox

@@ -107,6 +107,8 @@ See also:
 -   New @ref GL::Buffer::Buffer(Containers::ArrayView<const void>, BufferUsage)
     constructor for directly creating buffers filled with data.
 -   New @ref GL::Mesh::maxVertexAttributeStride() limit query
+-   Added a @ref GL::Shader::Shader(NoCreateT) constructor for consistency with
+    other OpenGL wrapper objects
 
 @subsubsection changelog-latest-new-math Math library
 

src/Magnum/GL/Shader.h

@@ -33,6 +33,7 @@
 #include <vector>
 #include <Corrade/Containers/ArrayView.h>
 
+#include "Magnum/Tags.h"
 #include "Magnum/GL/AbstractObject.h"
 #include "Magnum/GL/GL.h"
 
@@ -512,6 +513,21 @@ class MAGNUM_GL_EXPORT Shader: public AbstractObject {
          */
         explicit Shader(Version version, Type type);
 
+        /**
+         * @brief Construct without creating the underlying OpenGL object
+         *
+         * The constructed instance is equivalent to moved-from state. Useful
+         * in cases where you will overwrite the instance later anyway. Move
+         * another object over it to make it useful.
+         *
+         * This function can be safely used for constructing (and later
+         * destructing) objects even without any OpenGL context being active.
+         * However note that this is a low-level and a potentially dangerous
+         * API, see the documentation of @ref NoCreate for alternatives.
+         * @see @ref Shader()
+         */
+        explicit Shader(NoCreateT) noexcept: _type{}, _id{0} {}
+
         /** @brief Copying is not allowed */
         Shader(const Shader&) = delete;
 

src/Magnum/GL/Test/ShaderTest.cpp

@@ -34,15 +34,26 @@ namespace Magnum { namespace GL { namespace Test { namespace {
 struct ShaderTest: TestSuite::Tester {
     explicit ShaderTest();
 
+    void constructNoCreate();
     void constructCopy();
     void debugType();
 };
 
 ShaderTest::ShaderTest() {
-    addTests({&ShaderTest::constructCopy,
+    addTests({&ShaderTest::constructNoCreate,
+              &ShaderTest::constructCopy,
               &ShaderTest::debugType});
 }
 
+void ShaderTest::constructNoCreate() {
+    {
+        Shader shader{NoCreate};
+        CORRADE_COMPARE(shader.id(), 0);
+    }
+
+    CORRADE_VERIFY(true);
+}
+
 void ShaderTest::constructCopy() {
     CORRADE_VERIFY(!(std::is_constructible<Shader, const Shader&>{}));
     CORRADE_VERIFY(!(std::is_assignable<Shader, const Shader&>{}));