bring the gradient rendering down into SalGraphicsImpl & unx backend

Change-Id: Ia5d5e7d996b4a8818e73f3f741907ff9224f93da
remove useless define

Change-Id: I3033748cac045cd6e9595fadc84c65b21128b39d
move defines to the top of the file

Change-Id: Ia76fd2b1f9d7a9056425c760320f6b87965a2012