glsl: Create and use a has_explicit_attrib_location() helper.

glsl: Create and use a has_explicit_attrib_location() helper.

Explicit attribute locations are supported with GLSL 3.30, GLSL ES 3.00,
or "#extension GL_ARB_explicit_attrib_location: enable".  Using a helper
function makes it easy to check for this.

This enables support in GLSL 3.30, which was previously missing.

Previously, we overrode the extension enable flag for ES 3.00.  This is
not robust against a shader such as:

   #version 330
   #extension GL_ARB_explicit_attrib_location : disable

Disabling extensions should not remove core language functionality.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>