[client] egl: properly use OpenGL ES

Instead of using the desktop <GL/gl.h>, we properly use the OpenGL ES 3.x
headers. Also, we now use GL_EXT_buffer_storage for MAP_PERSISTENT_BIT_EXT
and MAP_COHERENT_BIT_EXT as the core versions are only available in desktop
OpenGL 4.4. Similarly, we need GL_EXT_texture_format_BGRA8888 for GL_BGRA_EXT
as GL_BGRA is desktop-only.

client/renderers/EGL/splash.c

@@ -26,7 +26,7 @@
 #include "shader.h"
 #include "model.h"
 
-#include <GL/gl.h>
+#include <GLES3/gl3.h>
 #include <stdlib.h>
 #include <string.h>
 #include <math.h>