OpenGL2: Disable the VAO cache by default

The VAO surface cache uses glBufferSubData and triggers a very slow
path in some GLES implementations. Specifically I have observed 10x
frame times under Emscripten with ANGLE/Metal on macOS and with Mali
on Android.

Co-authored-by: Zack Middleton <zack@cloemail.com>
Co-authored-by: WofWca <wofwca@protonmail.com>

code/renderergl2/tr_local.h

@@ -1848,6 +1848,8 @@ extern	cvar_t	*r_printShaders;
 
 extern cvar_t	*r_marksOnTriangleMeshes;
 
+extern cvar_t *r_vaoCache;
+
 //====================================================================
 
 static ID_INLINE qboolean ShaderRequiresCPUDeforms(const shader_t * shader)