GLSL: rename all identifiers prefixed with gl_.

In GLSL, all identifiers beginning with gl_ are reserved (not just those
explicitly named in the spec), and so any found in WGSL must be renamed.

Bug: tint:1304
Change-Id: I92ed7ec674620f67775378ecb8debcfdb4b5bbb4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/69701
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/transform/renamer.cc

@@ -1309,7 +1309,8 @@ Output Renamer::Run(const Program* in, const DataMap& inputs) {
                 kReservedKeywordsGLSL,
                 kReservedKeywordsGLSL +
                     sizeof(kReservedKeywordsGLSL) / sizeof(const char*),
-                name_in)) {
+                name_in) &&
+            name_in.compare(0, 3, "gl_")) {
           // No match, just reuse the original name.
           return ctx.dst->Symbols().New(name_in);
         }