Use GL_QUADS for flushing a quad batch

Instead of using GL_TRIANGLES and uploading the indices every time, it
now uses GL_QUADS instead on OpenGL. Under GLES it still uses indices
but it uses the new cogl_vertex_buffer_indices_get_for_quads function
to avoid uploading the vertices every time.

This requires the _cogl_vertex_buffer_indices_pointer_from_handle
function to be exposed privately to the rest of Cogl.

The static_indices array has been removed from the Cogl context.

common/cogl-vertex-buffer-private.h

@@ -161,5 +161,9 @@ typedef struct _CoglVertexBuffer
 
 } CoglVertexBuffer;
 
+CoglVertexBuffer *_cogl_vertex_buffer_pointer_from_handle (CoglHandle handle);
+CoglVertexBufferIndices *
+     _cogl_vertex_buffer_indices_pointer_from_handle (CoglHandle handle);
+
 #endif /* __COGL_VERTEX_BUFFER_H */