Ignore converting useless images

[codewario]

Romhacks are generally identifiable in libretro-thumbnails via the filename. These are useless until Analogue allows either (or both):

• unrecognized CRC IDs to display the Game Details screen on custom carts
• OpenFPGA cores to read and display Library images for loaded ROMs

e-Reader cards for the GBA also fall into a similar bucket.

Removing these will clean up verbose output a bit more as well as reduce the processing time and library size.

[codewario]

This has been completed. An md file was created to track explicit special cases and has also been codified in the script to either ignore images useless (today) on the Pocket or redirect one of the few missing entries that is actually due to a naming mismatch on the libretro-thumbnails side.