glossary: add stride/pitch
Linux DRM uses a definition extended to block-based non-linear layouts. See also:
- wayland/weston!695 (comment 1151711)
- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/drm_fourcc.c?h=v6.0#n389
Also add a link to drm_fourcc.h for "modifier".