gl-renderer: Always initialize variable

clang 17 complains that fourcc in gl_renderer_fill_buffer_info() is uninitialized in the default case, because it fails to recognize that if hit, that case will assert(0). To get rid of this complaint, we can just apply clang's suggestion and initialize the variable with 0 when declaring it.

Signed-off-by: Max Ihlenfeldt max@igalia.com