Based on a patch by Massimo.

Move the entire image-space/screen-space transformation logic from
gimp_display_shell_render() to gimp_display_shell_draw_image(), so
that the former works entirely in image space, and do the chunking
and clipping in screen-space, making sure that image-space chunks
are never larger than
GIMP_DISPLAY_RENDER_BUF_WIDTH x GIMP_DISPLAY_RENDER_BUF_HEIGHT,
even when the window's scale factor is greater than 1.

Add a GIMP_BRICK_WALL environment variable, which, when set, shows
the screen-space chunk bounds.