glsl: use linked shaders rather than compiled shaders

glsl: use linked shaders rather than compiled shaders

At this point there is no reason not to be using the linked shaders,
using the linked shaders should be faster and will make things simpler
for upcoming shader cache work.

The previous variable name suggests the linked shaders were intended
to be used here anyway.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>