add linear/point screen filter option, add fill mode aspect 4:3 or 16:10 options

[rsn8887]

Added option to turn bilinear screen smoothing on or off
Fixed wrong aspect ratio in fill mode
Added choice of 4:3 or 16:10 aspect ratio in fill mode

[rsn8887]

In the meantime, I realized that PRBoom in Retroarch also works very nicely and with perfect music.

You have to copy a few files into some directory:
doom.wad (official)
doom2.wad (official)
prboom.wad from here
.mp3 versions of all the music tracks (unzipped) from here, renamed. All mp3 filenames have to have the leading "d_" removed from their filenames to make it work.

Then start Retroarch, load content, select your folder and select doom.wad or doom2.wad, and play Doom with music!