GPUContext: Move GPUContext to gpu::Context for more consistency

This makes the GPUContext follow the same naming convention as the rest
of the module.

Also add a static getter for extra bonus style (no need for casts):
- Context::get()
- GLContext::get()

source/blender/gpu/opengl/gl_vertex_buffer.cc

@@ -52,7 +52,7 @@ void GLVertBuf::release_data(void)
 
 void GLVertBuf::duplicate_data(VertBuf *dst_)
 {
-  BLI_assert(GPU_context_active_get() != NULL);
+  BLI_assert(GLContext::get() != NULL);
   GLVertBuf *src = this;
   GLVertBuf *dst = static_cast<GLVertBuf *>(dst_);
 
@@ -82,7 +82,7 @@ void GLVertBuf::upload_data(void)
 
 void GLVertBuf::bind(void)
 {
-  BLI_assert(GPU_context_active_get() != NULL);
+  BLI_assert(GLContext::get() != NULL);
 
   if (vbo_id_ == 0) {
     glGenBuffers(1, &vbo_id_);