VolumeGC: Get rid of banner pointer casts

Prefer reading the data into the reified struct instead.

In the past, only GCZ was shown as compressed, not CISO and WBFS.

Fix reading Wii FST size (for real this time)

04fcb72 fixed an issue with reading the Wii FST size, but I found a second
issue when working on PR #2820 - the size must be shifted left by 2.
DiscScrubber and Boot already do this correctly using separate code.

GC games with long names store two variations of the name in
opening.bnr. This makes the shorter of those names available.
For volumes other than GC discs, prefer_long is ignored.

ISOFile and GameFile were using IsWiiDisc() and IsWadFile() to set
an enum value. The volume might as well return an enum directly.

I increased the Qt CACHE_REVISION because m_platform now is saved as u32
instead of int, but increasing the wx CACHE_REVISION is not necessary.