GPUState: Encapsulate glFlush and glFinish inside the GLContext

Part of the Vulkan task T68990

Isolate the few remaining gl functions.

source/blender/gpu/intern/gpu_context_private.hh

@@ -76,6 +76,12 @@ struct GPUContext {
 
   virtual void activate(void) = 0;
   virtual void deactivate(void) = 0;
+
+  /* Will push all pending commands to the GPU. */
+  virtual void flush(void) = 0;
+  /* Will wait until the GPU has finished executing all command. */
+  virtual void finish(void) = 0;
+
   virtual void memory_statistics_get(int *total_mem, int *free_mem) = 0;
 
   bool is_active_on_thread(void);

source/blender/gpu/intern/gpu_state.cc

@@ -295,12 +295,12 @@ bool GPU_mipmap_enabled(void)
 
 void GPU_flush(void)
 {
-  glFlush();
+  GPU_context_active_get()->flush();
 }
 
 void GPU_finish(void)
 {
-  glFinish();
+  GPU_context_active_get()->finish();
 }
 
 void GPU_unpack_row_length_set(uint len)

source/blender/gpu/opengl/gl_context.cc

@@ -160,6 +160,22 @@ void GLContext::deactivate(void)
 
 /** \} */
 
+/* -------------------------------------------------------------------- */
+/** \name Flush, Finish & sync
+ * \{ */
+
+void GLContext::flush(void)
+{
+  glFlush();
+}
+
+void GLContext::finish(void)
+{
+  glFinish();
+}
+
+/** \} */
+
 /* -------------------------------------------------------------------- */
 /** \name Safe object deletion
  *

source/blender/gpu/opengl/gl_context.hh

@@ -97,6 +97,10 @@ class GLContext : public GPUContext {
 
   void activate(void) override;
   void deactivate(void) override;
+
+  void flush(void);
+  void finish(void);
+
   void memory_statistics_get(int *total_mem, int *free_mem) override;
 
   static inline GLStateManager *state_manager_active_get()