Remove output->previous_damage from weston_output

In !321 (merged) we've removed the need of previous_damage member from struct weston_output for frame_signal listeners. Now it's time to decide what to do with output->previous_damage in the DRM backend when using Pixman renderer. This way we'd be able to remove previous_damage from weston_output. We have more than one option here, as mentioned in #256 (closed):

One easy option would be to move it to the drm_output structure. Other more complicated options would also be available.

We've decided to open this issue to discuss what is the best option here.

cc: @daniels