gl-renderer: add BUFFER_TYPE_SOLID
gl-renderer: add BUFFER_TYPE_SOLID

Add a new buffer type identifying the solid color contents which do not
have a real buffer.

Solid color surfaces now pretend to have 1x1 pixel content data.

This helps the future surface_get_data_size() implementation.

Signed-off-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>
Tested-by: Nobuhiko Tanibata <92313f26b3566e64b50391ce0051fc0e1f775402@xddp.denso.co.jp>
Reviewed-by: Daniel Stone <62e7e355d5d8400fa8384448796af0bf6be4ffae@collabora.com>

Tarnyko/weston-xdg_surface_present,Gnurou/weston,xorgy/weston,udoprog/weston,udoprog/weston,Fantu/compositor-spice,mchalupa/weston,sir-murray/weston,krezovic/weston,Tarnyko/weston-xdg_surface_present,sir-murray/weston,giucam/weston,Fantu/compositor-spice,xorgy/weston,Gnurou/weston,eyolfson/weston,jonnylamb/weston,krezovic/weston,mchalupa/weston,jonnylamb/weston,kwm81/weston,giucam/weston,Fantu/compositor-spice,eyolfson/weston,kwm81/weston