Add desert_main

[johannesHHM]

The hardest part was trying to get the shadow on all of the "main" tiles to match up, and look like the original. The originals shadows do not tile great when zoomed in, I think mine are a bit better, but still not perfect (this mostly an export bug, they line up fine in the svg)

Pixel diff of remake and original, the shadowy-areas have few perfect pixels.

compare mapres/desert_main.png /usr/share/ddnet/data/mapres/desert_main.png -compose src diff.png

A couple of bugs that I fixed / didn't replicate:

• tile 23 has some transparency inside of it
• tile 98's bottom line is a slightly different shade of brown then the other tiles it should tile with
• tile 136's wire shadow does not line up with the other wire tiles
• tile 22 has some light sand pixels coming out where it shouldn't (probably export bug)
• all the wall background inner corner tiles light shade line don't line up with the other tiles it tiles with

closes #55

[Jupeyy]

@ChillerDragon file name here is wrong too like in #53 if u still motivated for the CI xd

@johannesHHM looks good, thanks