Use sceKernelDcacheWritebackAll() as a hint only.
Return errors for Dcache funcs, correctly.
Recheck writebackall'd textures more frequently.

Since we're dealing with texture upload speed (time), basing this off
number of times invalidated (also time usually) seems to make sense.

This helps lazy games that just use writebackall and expect their
graphics to update.
Update tests.
Merge pull request #337 from unknownbrackets/texcache

Do more texcache checking for sceKernelDcacheWritebackAll()