Add Flip Option in raylib_util.nim

[Technisha]

Hello! I have been testing this out for a little while now and am now planning to make a little game, but i'm facing an issue because Textures seem to not have a simple way to flip them (If the character is moving one way), so is it possible if this feature can be implemented? Thanks! (If i figure out how to do this, i'll probably create a PR)

[Technisha]

Welp, i'm stupid because this was actually bugging me for about half an hour, buuuut i got it done! It can easily be implemented like this (look below), only issue is, it doesn't have any way of saying if it's flipped

proc TextureFlipH(texture:var Texture2D):Texture2D =
  texture.width = texture.width * -1
  return texture

proc TextureFlipV(texture:var Texture2D):Texture2D =
  texture.height = texture.height * -1
  return texture