frontend: Use inverted use_gles as a fallback for GL initialization. (#7117)

src/core/frontend/emu_window.h

@@ -77,6 +77,11 @@ class GraphicsContext {
 public:
     virtual ~GraphicsContext();
 
+    /// Checks whether this context uses OpenGL ES.
+    virtual bool IsGLES() {
+        return false;
+    }
+
     /// Inform the driver to swap the front/back buffers and present the current image
     virtual void SwapBuffers(){};