showimg is a funny little image viewer / overlay
- escape: close window
- left mouse button: move window, or resize it at its border
- right click: open the OS context menu for the window
- middle click (hold): select a region to zoom into
- backspace: reset zoom region
- T: toggle window background for transparent images (transparent, light checkerboard, dark checkerboard)
- fix aspect-aware window resize logic on X11
- mipmaps and SPD
- animations
- there are no good image diff viewers, this could become one
- HDR question mark (I have no use for this, I neither have HDR images nor monitors)