Add support for decoding PVR textures

[codywatts]

Textures encoded with PVRTC4BPP (0xB) and PVRTC2BPP (0xC) are recognized by the Puzzle & Dragons Texture Tool, but they cannot actually be decoded.

(Thanks to @Raijinili for alerting me to this issue!)

[kernigh]

I am lucky; I found no PVRTC textures in my .bc files while using this Texture Tool. I suspect that Puzzle & Dragons did not use PVRTC on my device. My .bc files are from late 2017; they include monsters from Halloween 2017 but not from Christmas 2017. I run the Texture Tool with Python 3.6.4 on OpenBSD 6.3, and it successfully extracts the images of my most favorite monsters.

[Raijinili]

@kernigh PVRTC is used for the iOS version, and possibly certain Android devices. App versions use different formats because different GPUs and OSes are better at using those formats. A given app version will presumably use the same (or related) format for all of its textures.