Provide a better R/W base PixelBuffer class

Clearly separates the read API from the write API
and also from actual implementation.
Only grab the regions we actually need

Allows pixel buffers that do damage tracking to get
proper information.
Add more 8 bpp conversion tests
Put some space between the groups of tests
Use lookup tables for pixel upconversion

You have to loop many times when converting e.g. 2 bits to 8,
so have a couple of lookup tables to speed things up. We still
use bit replication to get from 8 bits to 16 bits though.
Add ability to directly convert between two pixel formats

This is a lot easier and cheaper than having to set up a complete
PixelTransformer object.
Add optimised buffer conversion

Handles the common cases when the target or source are in
the preferred 888 format.

If one of the buffers is not 888, then it must also be properly
aligned (which is commonly the case). Performance is now in many
cases on par with PixelTransformer.
Test results comparing PixelTransformer and bufferFromBuffer()
Add convenience functions to ModifiablePixelBuffer

Allows you to modify the buffer with data in a
different pixel format.
Add convenience variant of getImage()
Add helper class for a rendered cursor

Add a magical cursor framebuffer class for handling when you
want to render the cursor on the server side. Keeps the cursor
specific magic in one contained place.
Allow PixelBuffers to be const
Store the mouse cursor in the ConnParams object

Like we do for everything else. This also gets rid
of the callback, which is a bit out of place
compared to everything else.
Use PixelBuffer objects as the interface for encoders and decoders

This avoid a lot of unnecessary middle men. This also pushes the
responsibility for pixel format conversion into the encoders and
decoders. The new bufferFromBuffer() is used for direct conversion,
rather than PixelTransformer/TransImageGetter.
Remove PixelTransformer and TransImageGetter

There is nothing using these classes anymore, so remove the clutter.
Merge branch 'nopixeltrans' of https://github.com/CendioOssman/tigervnc