MAKESCENE: fix case for original image missing

Checking for a NULL original image seems more reasonable here.
style fix: use NULL for null pointers
style fix: don't use RefPtr::get()
Merge pull request #49 from Waldteufel/fix-original-missing

MAKESCENE: fix case for original image missing
MVE: Fixed TriangleMesh::delete_vertices()
MATH: Added MATH_SQRT_2PI, changed MATH_SQRT1_2 to MATH_1_SQRT_2, changed MATH_2_SQRTPI to MATH_2_SQRT_PI
MATH: Optimization for math::algo::fastpow()
Merge pull request #50 from Waldteufel/fix-style

Style fixes