In GimpDisplayShell, scale the render cache by the window's scale
factor, and render its content in device pixels, instead of scaled
application pixels.  When painting the cache to the screen, unscale
the cairo context by the same factor, so that it's painted in the
native resolution.  Note that the various
gimp_display_shell_render_foo() functions still speak in
application pixels, and the scaling happens internally in
gimp_display_shell_render().

Aside from rendering at native resolution on HiDPI, this also fixes
an issue where grid-like display artifacts would appear when the
render cache was not fully validated due to the non-native scaling.