added preserveStartFrame attribute to AlembicNode: it will preserve the start frame invariant when modifying play back speed
added preserveStartFrame attribute to AbcShape: it will preserve the start frame invariant when modifying play back speed
compute transform self bounds as the sum of all children self bounds
Simplified draw visitors and select visitor into a single class
Perfected frustum culling algorithm
Finer culling when picking building the frustum from OpenGL state
Merge branch 'marza' of github.com:gatgui/Alembic into marza