glsl: Use a consistent technique for tracking link success/failure.

glsl: Use a consistent technique for tracking link success/failure.

This patch changes link_shaders() so that it sets prog->LinkStatus to
true when it starts, and then relies on linker_error() to set it to
false if a link failure occurs.

Previously, link_shaders() would set prog->LinkStatus to true halfway
through its execution; as a result, linker functions that executed
during the first half of link_shaders() would have to do their own
success/failure tracking; if they didn't, then calling linker_error()
would add an error message to the log, but not cause the link to fail.
Since it wasn't always obvious from looking at a linker function
whether it was called before or after link_shaders() set
prog->LinkStatus to true, this carried a high risk of bugs.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>