For comments: limiting the thumbnail cache sizes to < 640x480 to better support large screens

Here's a quick hack that made darktable work much better in my current setup (limited memory, large screen). I set the DT_MIPMAP_3 size to 640x480 max, whatever the max_width and max_height was set to in the preferences. This gave me a larger number of slots for thumbnails while at the same time letting me fill the screen in darkroom and lighttable Z mode (partially fixing #9884).

Curious why this couldn't be the default. It seems creating very large thumbnails doesn't make much sense past a certain point. Even at 4K resolutions 640x480 would give you 6x4 thumbnails if you filled the whole space. Since the lighttable ads borders it will give you even more