Fix direct lighting in MslProgram::bindLighting (#2781)

This changelist fixes a uniform name mismatch in `MslProgram::bindLighting`, which was causing the direct lighting term to be missing on MacOS, as reported in #2769.

In the MSL rendering path, uniforms are stored with a `GlobalContext` struct prefix, and the `hasUniform` and `bindUniform` methods correctly resolve these prefixed names through the global uniform name map.

Author: jstone-lucasfilm

source/MaterialXRenderMsl/MslPipelineStateObject.mm

@@ -687,8 +687,7 @@ int GetStrideOfMetalType(MTLDataType type)
 
     // Set the number of active light sources
     size_t lightCount = lightHandler->getLightSources().size();
-    auto numActiveLightSourcesInput = uniformList.find(HW::NUM_ACTIVE_LIGHT_SOURCES);
-    if (numActiveLightSourcesInput == uniformList.end())
+    if (!hasUniform(HW::NUM_ACTIVE_LIGHT_SOURCES))
     {
         // No lighting information so nothing further to do
         lightCount = 0;