glsl/linker: validate attribute aliasing before optimizations

glsl/linker: validate attribute aliasing before optimizations

Patch does a 'dry run' of assign_attribute_or_color_locations before
optimizations to catch cases where we have aliasing of unused attributes
which is forbidden by the GLSL ES 3.x specifications.

We need to run this pass before unused attributes may be removed and with
attribute binding information from program, therefore we re-use existing
pass in linker rather than attempt to write another one.

This fixes WebGL2 test 'gl-bindAttribLocation-aliasing-inactive' and
Piglit test 'gles-3.0-attribute-aliasing'.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106833
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>