9a879a7616
This fixes a mistake in commit 637728dd89d51bc28 that was meant to fix yv12/i420 support but when the patch was updated based on review to use COGL_PIXEL_FORMAT_A instead of COGL_PIXEL_FORMAT_G the corresponding glsl code wasn't also updated to sample the .a component. (cherry picked from commit a02c55105c705471866df2d306879d8616289f2a)