Gawain: stricter lookup of builtin uniforms

UNIFORM_NONE should never match a valid uniform (builtin or custom).

The logic for UNIFORM_CUSTOM was just wrong, since it returned the first custom uniform. This function should only accept builtin (non-custom) uniforms.

intern/gawain/src/shader_interface.c

@@ -273,6 +273,11 @@ const ShaderInput* ShaderInterface_uniform(const ShaderInterface* shaderface, co
 
 const ShaderInput* ShaderInterface_builtin_uniform(const ShaderInterface* shaderface, BuiltinUniform builtin)
 	{
+#if TRUST_NO_ONE
+	assert(builtin != UNIFORM_NONE);
+	assert(builtin != UNIFORM_CUSTOM);
+#endif
+
 	// look up by enum, not name
 	for (uint32_t i = 0; i < shaderface->uniform_ct; ++i)
 		{