Explicitly set read buffer size to 8k in default IO handler.

Some file systems were causing the buffer size to be as large as 512k
per file handle and this was using an inordinate amount of memory.  8k
seems plenty and can be tuned in PtexIO.h if desired.
Use float precision (avoid float/double conversions).
Remove LOD bias on bilinear filter.
Renamed formal parameters to silence -Wshadow downstream