Apparently on X11 the maximum icon size is 128x128. 256x256 is too
large for the X11 protocol because the X server unserializes the icons
using "unsigned long" which is 64 bits on Linux. So we have to use
64bits per pixel instead of 32, with 32bits padded to 0.
While there I also got rid of the kitty.rgba file replacing it with a
128x128 PNG file.
Fixes#3260
