sdl_layer <-
blend: {blend,add,mod,mul,none}How the texture is combined with what is below it, using the blend modes
of SDL. blend (default) is normal alpha blending.
add adds the colours, which makes glows, light, fire and
particles. mod multiplies the colours below with those of
the texture, ignoring its transparency. mul does the same,
but keeps what is below where the texture is transparent, e.g.,
for shadows.
none replaces what is below by the texture, ignoring sdl_layer<-alpha
and transparency; it is only useful for opaque textures.
