[Pixman] [bits] Some cleanups of fetchers, plus flags for bits-image

[Søren Sandmann]

The following patches contain some cleanups of the way the scanline
fetching works for bits images. By removing the support for alpha map
recursion, there is no longer any need to distinguish between 'raw'
and non-raw scanline fetchers.

Instead there is only one type of scanline fetcher, and only two
function pointers in the image struct: one for 32 bit fetching and one
for 64 bits.

This then allows the various types of fetchers to be stored in an
array indexed by flags, similar to how the fast paths are stored.

Soren