* split up FMultiPatchTexture into a builder class and the actual image source.
* since images can now be referenced by multiple textures the old redirection mechanism has been removed. It can be done better and less intrusive now. Simple single patch textures already directly reference the underlying patch image now.
* allocate all image source related data from a memory arena. Since this is all static this makes it a lot easier to free this in bulk.
Previously it tried to copy all patches of composite sub-images directly onto the main image.
This caused massive complications throughout the entire true color texture code and made any attempt of caching the source data for composition next to impossible because the entire composition process operated on the raw data read from the texture and not some cacheable image. While this may cause more pixel data to be processed, this will be easily offset by being able to reuse patches for multiple textures, once a caching system is in place, which even for the IWADs happens quite frequently.
Removing the now unneeded arguments from the implementation also makes things a lot easier to handle.
* it's no longer the main texture objects managing the pixel buffer but FSoftwareTexture.
* create proper spans for true color textures. The paletted spans only match if the image does not have any translucent pixels.
* create proper warp textures instead of working off the paletted variants.
As a side effect, caching of pixel buffers for texture composition is temporarily disabled, as it management of texture redirections. These things will be reimplemented once things progress further. The existing methods here had their share of serious issues that should be fixed.
This was done mainly to reduce the amount of occurences of the word FTexture but it immediately helped detect two small and mostly harmless bugs that were found due to the stricter type checks.
Now it is no longer necessary to provide specially set up textures for rendering shaded decals, they can use any PNG texture now that contains a proper red channel.
Handling of the alPh chunk has been removed as a result as it in no longer needed.
