gl: optimize glcolorbalance by precomputing shader math

gl: optimize glcolorbalance by precomputing shader math

Precompute the rgb -> yuv conversion and color balance adjustment math
so that the shader does minimal work per pixel.

Merging these 15+ steps into 3 steps let us jump from choppy
360p video to smooth 720p video on our underpowered embedded system.

If we can remove the clamp() step inside the shader, or apply it after
rgba
conversion, there are more performance benefits to reap. But I am not
sure
what the side effects will be in that case.

<https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/893>

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/893>