cogl-gst: video-sink: fix YV12/I420 support

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)
cogl2: add boilerplate to define fundamental types
pipeline: add introspection support
texture: add introspection support
texture-2d: add introspection support
texture-2d-sliced: add introspection support
texture-3d: add introspection support
texture-rectangle: add introspection support
atlas-texture: add introspection support
sub-texture: add introspection support
bitmap: add introspection support
color: add introspection support
framebuffer: add introspection support
onscreen: add introspection support
context: add introspection support
renderer: add introspection support
display: add introspection support
primitive: add introspection support
attribute: add introspection support
attribute-buffer: add introspection support