PtexFilter.cpp: add explicit casts to silence -Wconversion warnings

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge pull request #10 from davvid/conversion-warnings

PtexFilter.cpp: add explicit casts to silence -Wconversion warnings
Fix regression in triangle filter from commit 194207.
Merge branch 'master' into v2.1-beta