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>