gl-renderer: Use helper for conversion to EGL rects

eglSwapBuffersWithDamage has to convert a damage region from Weston's
global co-ordinate space, into the co-ordinate space for EGL rendering
into a buffer for that output.

The conversion from the global co-ordinate space in logical pixels to
the output space in buffer pixels is slightly long and error-prone,
involving translating by the output's offset within the global
co-ordinate space, multiplying by output scale, and also translating to
allow for any borders we paint around the output.

After this is done, we need to flip the co-ordinates in the Y axis to
account for the lower-left-origin co-ordinate space used by EGL.

Since we want to reuse this for partial_update, but using a different
source region, extract this conversion into a well-commented helper we
can reuse.

Signed-off-by: Daniel Stone <daniels@collabora.com>