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_uniform_buffer.cc

@@ -56,7 +56,7 @@ GLUniformBuf::~GLUniformBuf()
 
 void GLUniformBuf::init(void)
 {
-  BLI_assert(GPU_context_active_get());
+  BLI_assert(GLContext::get());
 
   glGenBuffers(1, &ubo_id_);
   glBindBuffer(GL_UNIFORM_BUFFER, ubo_id_);
@@ -112,7 +112,7 @@ void GLUniformBuf::bind(int slot)
 
 #ifdef DEBUG
   BLI_assert(slot < 16);
-  static_cast<GLContext *>(GPU_context_active_get())->bound_ubo_slots |= 1 << slot;
+  GLContext::get()->bound_ubo_slots |= 1 << slot;
 #endif
 }
 
@@ -122,7 +122,7 @@ void GLUniformBuf::unbind(void)
   /* NOTE: This only unbinds the last bound slot. */
   glBindBufferBase(GL_UNIFORM_BUFFER, slot_, 0);
   /* Hope that the context did not change. */
-  static_cast<GLContext *>(GPU_context_active_get())->bound_ubo_slots &= ~(1 << slot_);
+  GLContext::get()->bound_ubo_slots &= ~(1 << slot_);
 #endif
   slot_ = 0;
 }