glsl: Add gl_shader_program::UniformLocationBaseScale

glsl: Add gl_shader_program::UniformLocationBaseScale

This is used by _mesa_uniform_merge_location_offset and
_mesa_uniform_split_location_offset to determine how the base and offset
are packed.  Previously, this value was hard coded as (1U<<16) in those
functions via the shift and mask contained therein.  The value is still
(1U<<16), but it can be changed in the future.

The next patch dynamically generates this value.

NOTE: This is a candidate for stable release branches.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-and-tested-by: Chad Versace <chad.versace@linux.intel.com>