Bind some default VAO in case we are on core profile with ARB_VAO disabled.

Otherwise it's not possible to render anything. The original behavior
was accepted by NVidia, but it's not accepted by Mesa, e.g.

src/Magnum/Context.cpp

@@ -781,7 +781,10 @@ std::vector<std::string> Context::extensionStrings() const {
 
 #ifndef MAGNUM_TARGET_GLES
 bool Context::isCoreProfile() {
-    Implementation::ContextState& state = *_state->context;
+    return isCoreProfileInternal(*_state->context);
+}
+
+bool Context::isCoreProfileInternal(Implementation::ContextState& state) {
     Implementation::ContextState::CoreProfile& value = state.coreProfile;
 
     if(value == Implementation::ContextState::CoreProfile::Initial)

src/Magnum/Context.h

@@ -523,6 +523,10 @@ class MAGNUM_EXPORT Context {
         bool isDriverWorkaroundDisabled(const std::string& workaround);
         Implementation::State& state() { return *_state; }
 
+        /* This function is called from MeshState constructor, which means the
+           state() pointer is not ready yet so we have to pass it directly */
+        MAGNUM_LOCAL bool isCoreProfileInternal(Implementation::ContextState& state);
+
     private:
         explicit Context(NoCreateT, Int argc, const char** argv, void functionLoader());
 

src/Magnum/Implementation/MeshState.cpp

@@ -33,7 +33,7 @@
 
 namespace Magnum { namespace Implementation {
 
-MeshState::MeshState(Context& context, std::vector<std::string>& extensions): currentVAO(0)
+MeshState::MeshState(Context& context, ContextState& contextState, std::vector<std::string>& extensions): currentVAO(0)
     #ifndef MAGNUM_TARGET_GLES2
     , maxElementIndex{0}, maxElementsIndices{0}, maxElementsVertices{0}
     #endif
@@ -159,6 +159,24 @@ MeshState::MeshState(Context& context, std::vector<std::string>& extensions): cu
     #endif
     else vertexAttribDivisorImplementation = nullptr;
     #endif
+
+    #ifndef MAGNUM_TARGET_GLES
+    /* If we are on core profile and ARB_VAO was explicitly disabled by the
+       user, we need to bind a default VAO so we are still able to draw things */
+    if(context.isExtensionDisabled<Extensions::GL::ARB::vertex_array_object>() && context.isCoreProfileInternal(contextState)) {
+        glGenVertexArrays(1, &defaultVAO);
+        glBindVertexArray(defaultVAO);
+    }
+    #else
+    static_cast<void>(contextState);
+    #endif
+}
+
+MeshState::~MeshState() {
+    #ifndef MAGNUM_TARGET_GLES
+    /* If the default VAO was created, we need to delete it to avoid leaks */
+    if(defaultVAO) glDeleteVertexArrays(1, &defaultVAO);
+    #endif
 }
 
 void MeshState::reset() {

src/Magnum/Implementation/MeshState.h

@@ -32,8 +32,11 @@
 
 namespace Magnum { namespace Implementation {
 
+struct ContextState;
+
 struct MeshState {
-    explicit MeshState(Context& context, std::vector<std::string>& extensions);
+    explicit MeshState(Context& context, ContextState& contextState, std::vector<std::string>& extensions);
+    ~MeshState();
 
     void reset();
 
@@ -56,6 +59,10 @@ struct MeshState {
     void(*multiDrawImplementation)(std::initializer_list<std::reference_wrapper<MeshView>>);
     #endif
 
+    #ifndef MAGNUM_TARGET_GLES
+    GLuint defaultVAO{}; /* Used on core profile in case ARB_VAO is disabled */
+    #endif
+
     GLuint currentVAO;
     #ifndef MAGNUM_TARGET_GLES2
     #ifndef MAGNUM_TARGET_WEBGL

src/Magnum/Implementation/State.cpp

@@ -66,7 +66,7 @@ State::State(Context& context, std::ostream* const out) {
     debug.reset(new DebugState{context, extensions});
     #endif
     framebuffer.reset(new FramebufferState{context, extensions});
-    mesh.reset(new MeshState{context, extensions});
+    mesh.reset(new MeshState{context, *this->context, extensions});
     #if !(defined(MAGNUM_TARGET_WEBGL) && defined(MAGNUM_TARGET_GLES2))
     query.reset(new QueryState{context, extensions});
     #endif