Fixes this GCC warning.
mask.c: In function ‘mask_layer_fill’:
mask.c:387:12: warning: variable ‘alpha_color’ set but not used
[-Wunused-but-set-variable]
Signed-off-by: Vinson Lee <vlee@freedesktop.org> Reviewed-by: José Fonseca <jfonseca@vmware.com>