VideoCommon: Start putting common texture decoding code in TextureDecoder_Common

This pulls all the duplicate code from TextureDecoder_Generic /
TextureDecoder_x64 out and puts it in a common file. Out custom font
used for debugging the texture cache is also pulled out and put in a
common "sfont.inc" file. At some point we should also combine this font
with the other six binary fonts we ship.
VideoCommon: Remove support for decoding to ARGB textures

The D3D / OGL backends only ever used RGBA textures, and the Software
backend uses its own custom code for sampling. The ARGB path seems to
just be dead code.

Since ARGB and RGBA formats are similar, I don't think this will make
the code more difficult to read or unable to be used as
reference. Somebody who wants to use this code to output ARGB can simply
modify the MakeRGBA function to put the shift at the other end.
TextureDecoder: Add some statics to some of our helper functions

I know these are already inline, but this makes it more clear that
they're helper functions to be used in this file only.
TextureDecoder: Rearrange header slightly

Put the two Decode APIs together.
TextureDecoder: Add an enum for the TLUT formats

Quick code cleanup. The enum names and values come from libogc.
TextureDecoder: Pass the TLUT address straight into the texture decoder

This removes the requirement for the TextureDecoder to have access to
global texture memory.
TextureDecoder: Simplify how the reference texture decoder works

Instead of having three separate functions and checking the tlutfmt in a
variety of places, just do it once in a helper method. This is already
for the slow path either in our Generic decoder or in our Software
renderer, so it doesn't matter that this is slower.

x64 will continue using the separate functions for speed.
TextureDecoder: Clean up the code style

For a long time, we've had ugly and inconsistent function names here as
helpers, names like "decodebytesRGB5A3rgba" which are absolutely
incomprehensible to understand. Fix this by introducing a new consistent
naming scheme, where the above function now becomes "DecodeBytes_RGB5A3".