This means that a tex_t object is passed in instead of just raw bytes
and width and height, but it means the texture can specify whether it's
flipped or uses BGR instead of RGB. This fixes the upside down
screenshots for vulkan.
QFS_NextFilename was renamed to QFS_NextFile to reflect the fact it now
returns a QFile pointer for the newly created file (as well as the
name). This necessitated updating WritePNG to take a file pointer
instead of a file name, with the advantage that WritePNGqfs is no longer
necessary and callers have much more control over the creation of the
file.
This makes QFS_NextFile much more secure against file system race
conditions and attacks (at least in theory). If nothing else, it will
make it more robust in a multi-threaded environment.
