gl-renderer: Track the buffer type in gl_surface_state

gl-renderer: Track the buffer type in gl_surface_state

Checking for gs->num_images for determining the previous buffer type
when attaching is not reliable. The number of images is never cleared
in the SHM path, so after a switch from an EGL buffer to SHM, every
following attach of an SHM buffer will happen with gs->num_images > 0,
and the code will assume the previous buffer was an EGL one.

Fix this by adding a buffer_type field to gl_surface_state.