COPY_SHADERS should only copy changed files.

[alandtse]

trafficstars

See failed attempt here #526

This is caused by the use of:

			COMMAND ${CMAKE_COMMAND} -E copy_directory "${AIO_DIR}/Shaders" "${DEPLOY_TARGET}/Shaders"

Which will touch EVERY shader and thus force EVERY SHADER TO RECOMPILE. This is inefficient.