GL: fix build on 32-bit Windows.

Now that the CI checks this I feel much safer.

doc/changelog.dox

@@ -229,6 +229,8 @@ See also:
 -   `FindMagnum.cmake` now properly recognizes an optional dependency between
     @ref DebugTools and @ref Trade on GL-less builds
 -   Various compiler warning fixes (see [mosra/magnum#406](https://github.com/mosra/magnum/pull/406))
+-   Added a 32-bit Windows build to the CI matrix to avoid random compilation
+    issues (see [mosra/magnum#421](https://github.com/mosra/magnum/issues/421))
 
 @subsection changelog-latest-bugfixes Bug fixes
 

src/Magnum/GL/Implementation/RendererState.h

@@ -43,17 +43,19 @@ struct RendererState {
     #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
     void(*minSampleShadingImplementation)(GLfloat);
     #endif
+    /* These are direct pointers to the GL functions, so need a __stdcall on
+       Windows to compile properly on 32 bits */
     #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
-    void (*patchParameteriImplementation)(GLenum, GLint);
+    void(APIENTRY *patchParameteriImplementation)(GLenum, GLint);
     #endif
     #if !(defined(MAGNUM_TARGET_WEBGL) && defined(MAGNUM_TARGET_GLES2))
-    void(*enableiImplementation)(GLenum, GLuint);
+    void(APIENTRY *enableiImplementation)(GLenum, GLuint);
-    void(*disableiImplementation)(GLenum, GLuint);
+    void(APIENTRY *disableiImplementation)(GLenum, GLuint);
-    void(*blendEquationiImplementation)(GLuint, GLenum);
+    void(APIENTRY *blendEquationiImplementation)(GLuint, GLenum);
-    void(*blendEquationSeparateiImplementation)(GLuint, GLenum, GLenum);
+    void(APIENTRY *blendEquationSeparateiImplementation)(GLuint, GLenum, GLenum);
-    void(*blendFunciImplementation)(GLuint, GLenum, GLenum);
+    void(APIENTRY *blendFunciImplementation)(GLuint, GLenum, GLenum);
-    void(*blendFuncSeparateiImplementation)(GLuint, GLenum, GLenum, GLenum, GLenum);
+    void(APIENTRY *blendFuncSeparateiImplementation)(GLuint, GLenum, GLenum, GLenum, GLenum);
-    void(*colorMaskiImplementation)(GLuint, GLboolean, GLboolean, GLboolean, GLboolean);
+    void(APIENTRY *colorMaskiImplementation)(GLuint, GLboolean, GLboolean, GLboolean, GLboolean);
     #endif
     #ifndef MAGNUM_TARGET_WEBGL
     Renderer::GraphicsResetStatus(*graphicsResetStatusImplementation)();