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.
TextureDecoder: Remove unused function
VideoCommon: Start putting common texture decoding code in TextureDecoder_Common
VideoCommon: Remove support for decoding to ARGB textures

We exclusively use RGBA textures now. In reality, this was the only code
path that was ever exercised.
TextureDecoder: Add some statics to some of our helper functions
TextureDecoder: Rearrange header slightly

Put the two Decode APIs together.
TextureDecoder: Specify enums for the TLUT formats

These 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".