PixelEngine: Remove unused AllowIdleSkipping and all references to it
TextureDecoder: Fix a RGBA/BGRA copy/paste typo

We were decoding to BGRA32 textures in our RGBA32 texture decoder. Since
this is the same for the BGRA32 decoder implementation, this is most
likely a copy/paste typo, rather than the texture actually being
bit-swapped. Fix this.

I'm not sure of any games that use the C14X2 texture format, so I'm not
sure this fixes any games, but it does make the code cleaner for when we
clean it up in the future, and merge some of these similar loops.
TextureDecoder: Remove unused function

GetPC_TexFormat was never used. It was added in commit d02426a, with the
only user being commented out code. The commented out code was later
removed in 9893122, but the implementation stayed.
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".
Merge pull request #774 from magcius/texdecode-cleanup

Clean up the TextureDecoder and some related things