vk: fix push constant implementation and test (#9588)

All stages share a push constant "block". We need to order
the fragment stage bytes to after the vertex stage if they
both exists.

FIXES=453776664

libs/filamat/src/shaders/CodeGenerator.cpp

@@ -963,6 +963,10 @@ utils::io::sstream& CodeGenerator::generateSpecializationConstant(utils::io::sst
     return out;
 }
 
+// Note that we've only introduced push constants to the vertex stage.  If we want to add push
+// constants to the fragment stage, in vulkan, we would have to offset the definition of the field
+// by the size of the constant struct in the vertex stage. This is due to vulkan having essentially
+// one block of memory for push constants that is shared across all stages).
 utils::io::sstream& CodeGenerator::generatePushConstants(utils::io::sstream& out,
         MaterialBuilder::PushConstantList const& pushConstants, size_t const layoutLocation) const {
     if (UTILS_UNLIKELY(pushConstants.empty())) {