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-sys.h

@@ -30,6 +30,14 @@
 #include "backends/platform/sdl/sdl-sys.h"
 #endif
 
+// We allow to force GL or GLES modes on compile time.
+// For this the USE_GLES_MODE define is used. The following values represent
+// the given selection choices:
+//  0 - Force OpenGL context
+//  1 - Force OpenGL ES context
+#define USE_FORCED_GL   (defined(USE_GLES_MODE) && USE_GLES_MODE == 0)
+#define USE_FORCED_GLES (defined(USE_GLES_MODE) && USE_GLES_MODE == 1)
+
 // On Tizen we include the toolchain's OpenGL file. This is something we
 // actually want to avoid. However, since Tizen uses eglGetProcAddress which
 // is not required to return valid function pointers to non OpenGL extension
@@ -59,10 +67,21 @@
 
 namespace OpenGL {
 
+enum ContextType {
+	kContextGL,
+	kContextGLES
+};
+
 /**
  * Description structure of the OpenGL (ES) context.
  */
 struct Context {
+	/** Whether the context is properly initalized or not. */
+	bool ready;
+
+	/** The type of the active context. */
+	ContextType type;
+
 	/**
 	 * Reset context.
 	 *