Slider: add a Destroy() call for a LinearGradient obj, in ClippedRender()
build: fix warnings (which show up when using latest Mono.Cairo)

The warnings fixed in this commit are:
- Replace usage of Color setter with the SetSourceColor() method
- Replace usage of Pattern setter with the SetSource() method
- Replace usage of Destroy() method, with the using{} block dispose syntax

This is required to be able to enable WarnAsErrors back again in Banshee.