Port to cross-platform API

[ForNeVeR]

Eventually, after fixing all of the migration issues, I want to make Bloodmasters cross-platform, using Raylib or Monogame or something else.

But before that, we need to make sure everything works properly on the old DirectX. This means the following issues have to be resolved:

• [x] #8
• [x] #14
• [x] #15
• [x] #16
• [x] #17

Also, the following infrastructure tasks have to be resolved:

• [x] #10
• [x] #11
• [x] #12
• [x] #13
• [x] #15
• [x] #19