egl/main: Add bits for EGL_KHR_mutable_render_buffer

A follow-up patch enables EGL_KHR_mutable_render_buffer for Android.
This patch is separate from the Android patch because I think it's
easier to review the platform-independent bits separately.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

src/egl/main/egldisplay.h

@@ -127,6 +127,7 @@ struct _egl_extensions
    EGLBoolean KHR_image;
    EGLBoolean KHR_image_base;
    EGLBoolean KHR_image_pixmap;
+   EGLBoolean KHR_mutable_render_buffer;
    EGLBoolean KHR_no_config_context;
    EGLBoolean KHR_partial_update;
    EGLBoolean KHR_reusable_sync;