shaderc: Flip gl_Position.y when building SPIR-V.

tools/shaderc/shaderc.cpp

@@ -1765,6 +1765,13 @@ namespace bgfx
 //									);
 						}
 
+						if (0 != spirv)
+						{
+							preprocessor.writef(
+								"\tgl_Position.y = -gl_Position.y; \\\n"
+								);
+						}
+
 						preprocessor.writef(
 							"\treturn _varying_"
 							);