Damage in presentation

[i509VCB]

In order to efficiently draw on the CPU, it may be desireable to reduce how many pixels are actually drawn.

Often this involves telling the display server what pixels were changed so that the display server can reuse it's existing framebuffer or perform a partial upload of the texture data to the gpu to save pci bandwidth.

[notgull]

Maybe this could be another function, set_pixels_with_damage, where the user also passes in a "damage" value?

[i509VCB]

Also remembered that we would probably need to tell the user the age of the buffer.

[ids1024]

Now that #65 is merged, we could have a present_with_damage() method.

This is natural enough on Wayland. Do other backends have a way to support this?