Issue #3781 - Display artifacts on HiDPI when render cache is invalidated
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.
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