glsl: fix interpolateAtXxx(some_vec[idx], ...) with dynamic idx

The dynamic index of a vector (not array!) is lowered to a sequence of
conditional assignments. However, the interpolate_at_* expressions
require that the interpolant is an l-value of a shader input.

So instead of doing conditional assignments of parts of the shader input
and then interpolating that (which is nonsensical), we interpolate the
entire shader input and then do conditional assignments of the interpolated
result.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit ca63a5ed)