Bug 1734027 - Ensure we mark GLFeature::draw_buffers_indexed as unsupported if symbol load fails. r=jgilbert

When loading the symbols for draw_buffers_indexed, we marked it as for
draw_buffers. This would mean that if draw_buffers_indexed loading
failed, and we checked for the feature support, it would indicate it is
supported even though it is backed by nothing.

Differential Revision: https://phabricator.services.mozilla.com/D136381

gfx/gl/GLContext.cpp

@@ -1290,7 +1290,7 @@ void GLContext::LoadMoreSymbols(const SymbolLoader& loader) {
             { (PRFuncPtr*) &mSymbols.fEnablei, {{ "glEnableiOES" }} },
             END_SYMBOLS
         };
-        fnLoadFeatureByCore(coreSymbols, extSymbols, GLFeature::draw_buffers);
+        fnLoadFeatureByCore(coreSymbols, extSymbols, GLFeature::draw_buffers_indexed);
     }
 
     if (IsSupported(GLFeature::get_integer_indexed)) {