GL: state reset needs to unbind PBOs when entering external code.

Qt is not PBO-aware and it messes with its state when resizing a window.

doc/changelog.dox

@@ -300,6 +300,9 @@ See also:
 -   @ref GL::DynamicAttribute can now describe matrix attributes similarly to
     @ref GL::Attribute, previously this was possible only by specifying each
     column vector separately
+-   Added @ref GL::Context::State::UnbindPixelBuffer that's now implicitly
+    included in @ref GL::Context::State::EnterExternal to make Magnum work
+    correctly with 3rd party code that's not aware of PBOs.
 
 @subsubsection changelog-latest-changes-math Math library
 

src/Magnum/GL/Context.cpp

@@ -1008,6 +1008,17 @@ Version Context::supportedVersion(std::initializer_list<Version> versions) const
 }
 
 void Context::resetState(const States states) {
+    #ifndef MAGNUM_TARGET_GLES2
+    /* Unbind a PBO (if any) to avoid confusing external GL code that is not
+       aware of those. Doing this before all buffer state is reset so we can
+       reuse the knowledge in our state tracker and unbind only if Magnum
+       actually bound a PBO before. */
+    if(states & State::UnbindPixelBuffer) {
+        Buffer::unbindInternal(Buffer::TargetHint::PixelPack);
+        Buffer::unbindInternal(Buffer::TargetHint::PixelUnpack);
+    }
+    #endif
+
     if(states & State::Buffers)
         _state->buffer->reset();
     if(states & State::Framebuffers)

src/Magnum/GL/Context.h

@@ -267,11 +267,23 @@ class MAGNUM_GL_EXPORT Context {
             /** Reset tracked buffer-related bindings and state */
             Buffers = 1 << 0,
 
+            #ifndef MAGNUM_TARGET_GLES2
+            /**
+             * Unbind currently bound PBO.
+             *
+             * Not all third-party code is aware of PBOs, and if a PBO is bound
+             * when Magnum transfers control to an unaware code, it can cause
+             * various issues with textures. This is a similar, but rarer,
+             * case to @ref State::MeshVao / @ref State::BindScratchVao.
+             */
+            UnbindPixelBuffer = 1 << 1,
+            #endif
+
             /** Reset tracked framebuffer-related bindings and state */
-            Framebuffers = 1 << 1,
+            Framebuffers = 1 << 2,
 
             /** Reset tracked mesh-related bindings */
-            Meshes = 1 << 2,
+            Meshes = 1 << 3,
 
             /**
              * Unbind currently bound VAO.
@@ -285,7 +297,7 @@ class MAGNUM_GL_EXPORT Context {
              * with @ref State::MeshVao included unbounds any currently bound
              * VAO to fix such case.
              */
-            MeshVao = 1 << 3,
+            MeshVao = 1 << 4,
 
             /**
              * Bind a "scratch" VAO on core profile.
@@ -298,40 +310,46 @@ class MAGNUM_GL_EXPORT Context {
              * Does nothing on compatibility profile and ES / WebGL platforms,
              * as using the default VAO is allowed there.
              */
-            BindScratchVao = 1 << 4,
+            BindScratchVao = 1 << 5,
 
             /** Reset tracked pixel storage-related state */
-            PixelStorage = 1 << 5,
+            PixelStorage = 1 << 6,
 
             /** Reset tracked renderer-related state */
-            Renderer = 1 << 6,
+            Renderer = 1 << 7,
 
             /** Reset tracked shader-related bindings */
-            Shaders = 1 << 7,
+            Shaders = 1 << 8,
 
             /** Reset tracked texture-related bindings and state */
-            Textures = 1 << 8,
+            Textures = 1 << 9,
 
             #ifndef MAGNUM_TARGET_GLES2
             /** Reset tracked transform feedback-related bindings */
-            TransformFeedback = 1 << 9,
+            TransformFeedback = 1 << 10,
             #endif
 
             /**
              * Reset state on entering section with external OpenGL code.
              *
              * Resets all state that could cause external code to accidentally
-             * modify Magnum objects. This includes only @ref State::MeshVao.
+             * modify Magnum objects. This includes @ref State::MeshVao and
-             * In some pathological cases you may want to enable
+             * @ref State::UnbindPixelBuffer. In some pathological cases you
-             * @ref State::BindScratchVao as well.
+             * may want to enable @ref State::BindScratchVao as well.
              */
-            EnterExternal = MeshVao,
+            EnterExternal = MeshVao
+                #ifndef MAGNUM_TARGET_GLES2
+                |UnbindPixelBuffer
+                #endif
+                ,
 
             /**
              * Reset state on exiting section with external OpenGL code.
              *
              * Resets Magnum state tracker to avoid being confused by external
-             * state changes. This resets all states.
+             * state changes. This resets all states, however
+             * @ref UnbindPixelBuffer is excluded as Magnum's state tracker
+             * will ensure no PBO is bound when calling related OpenGL APIs.
              */
             ExitExternal = Buffers|Framebuffers|Meshes|MeshVao|PixelStorage|Renderer|Shaders|Textures
                 #ifndef MAGNUM_TARGET_GLES2