LibGfx: Use Skia to speed up alpha conversions on Linux

[tcl3]

When profiling bbc.co.uk, this reduces the time spent in Bitmap::set_alpha_destructive() from 4.13% to 3.65%. This is not as much of a speed-up as I hoped for, but I thought I'd PR it anyway, since it is definitely faster.

The SkPixmap methods still use the CPU, ideally we should find a way to do this conversion on the GPU if possible.

EDIT: I was able to improve performance significantly by not attempting to alter opaque images.