Enhance the existing but unused display scaling (hidpi/retina) support
to work independently in x and y direction, and adjust the scaling
factors accordingly when dot-for-dot is off and xres != yres.

Increase GIMP_DISPLAY_RENDER_MAX_SCALE from 2.0 to 4.0 and adjust the
rendering chunk size dynamically so we never render chunks that do
not fit into the GimpDisplayXfer buffers.