glsl: handle same struct redeclaration (v2)

glsl: handle same struct redeclaration (v2)

This works around a bug in older version of UE4, where a shader
defines the same structure twice. Although we aren't sure this is correct
GLSL (it most likely isn't) there are enough UE4 based things out there
we should deal with this.

This drops the error to a warning if the struct names and contents match.

v1.1: do better C++ on record_compare declaration (Rob)
v2: restrict this to desktop GL only (Ian)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95005
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>