GL: don't use member function pointers in ContextState.

Since I'm touching this anyway. Making the internals better, one step at
a time.

src/Magnum/GL/Context.cpp

@@ -1129,7 +1129,7 @@ bool Context::isCoreProfileInternal(Implementation::ContextState& state) {
            that */
         if(_version < Version::GL320)
             value = Implementation::ContextState::CoreProfile::Compatibility;
-        else value = (this->*state.isCoreProfileImplementation)() ?
+        else value = state.isCoreProfileImplementation(*this) ?
             Implementation::ContextState::CoreProfile::Core :
             Implementation::ContextState::CoreProfile::Compatibility;
     }
@@ -1137,14 +1137,14 @@ bool Context::isCoreProfileInternal(Implementation::ContextState& state) {
     return value == Implementation::ContextState::CoreProfile::Core;
 }
 
-bool Context::isCoreProfileImplementationDefault() {
+bool Context::isCoreProfileImplementationDefault(Context&) {
     GLint value = 0;
     glGetIntegerv(GL_CONTEXT_PROFILE_MASK, &value);
     return value & GL_CONTEXT_CORE_PROFILE_BIT;
 }
 
-bool Context::isCoreProfileImplementationNV() {
-    auto extensions = extensionStrings();
+bool Context::isCoreProfileImplementationNV(Context& self) {
+    auto extensions = self.extensionStrings();
     return std::find(extensions.begin(), extensions.end(), "GL_ARB_compatibility") == extensions.end();
 }
 #endif

src/Magnum/GL/Context.h

@@ -881,8 +881,8 @@ class MAGNUM_GL_EXPORT Context {
         MAGNUM_GL_LOCAL void setupDriverWorkarounds();
 
         #ifndef MAGNUM_TARGET_GLES
-        MAGNUM_GL_LOCAL bool isCoreProfileImplementationDefault();
-        MAGNUM_GL_LOCAL bool isCoreProfileImplementationNV();
+        MAGNUM_GL_LOCAL static bool isCoreProfileImplementationDefault(Context& self);
+        MAGNUM_GL_LOCAL static bool isCoreProfileImplementationNV(Context& self);
         #endif
 
         void(*_functionLoader)(Context&){};

src/Magnum/GL/Implementation/ContextState.h

@@ -28,13 +28,6 @@
 #include "Magnum/Magnum.h"
 #include "Magnum/GL/GL.h"
 
-#ifdef _MSC_VER
-/* 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/GL/Context.h"
-#endif
-
 namespace Magnum { namespace GL { namespace Implementation {
 
 struct ContextState {
@@ -47,7 +40,7 @@ struct ContextState {
         Compatibility
     } coreProfile = CoreProfile::Initial;
 
-    bool (Context::*isCoreProfileImplementation)();
+    bool(*isCoreProfileImplementation)(Context&);
     #endif
 };