Buffer damage and scaling (attempt 2)

Created by: acrisci

Keep track of the scale of the buffer set by surface.set_buffer_scale.

Calculate the height and width of the texture from the buffer depending on the scale when it is attached.

Use buffer damage to determine the damage of the buffer when flushing damage.

Convert surface damage to buffer damage and union to the surface buffer_damage prior to flushing damage.

supercedes #65