OPENGL: Allow runtime specification of OpenGL mode.

Formerly, we required that the OpenGL mode was fixed at compile time. Now we
allow the code to work with whatever it is given at runtime.

It is still possible to force a context type on compile time.

backends/graphics/opengl/opengl-graphics.h

@@ -116,6 +116,16 @@ public:
 	virtual void grabPalette(byte *colors, uint start, uint num);
 
 protected:
+	/**
+	 * Whether an OpenGL (context) is active.
+	 */
+	bool isInitialized() const { return g_context.ready; }
+
+	/**
+	 * Whether an GLES context is active.
+	 */
+	bool isGLESContext() const { return g_context.type == kContextGLES; }
+
 	/**
 	 * Set up the actual screen size available for the OpenGL code to do any
 	 * drawing.
@@ -133,8 +143,9 @@ protected:
 	 *                           (this is used for the CLUT8 game screens).
 	 * @param defaultFormatAlpha The new default format with an alpha channel
 	 *                           (this is used for the overlay and cursor).
+	 * @param type               Type of the created context.
 	 */
-	void notifyContextCreate(const Graphics::PixelFormat &defaultFormat, const Graphics::PixelFormat &defaultFormatAlpha);
+	void notifyContextCreate(const Graphics::PixelFormat &defaultFormat, const Graphics::PixelFormat &defaultFormatAlpha, ContextType type);
 
 	/**
 	 * Notify the manager that the OpenGL context is about to be destroyed.
@@ -282,8 +293,10 @@ private:
 
 	/**
 	 * Initialize the active context for use.
+	 *
+	 * @param type Type of the context to initialize.
 	 */
-	void initializeGLContext();
+	void initializeGLContext(ContextType type);
 
 protected:
 	/**