Fix non-BMesh containing .blend import

BlenderBMeshConverter was erroneously asserting that it contained valid
mesh upon construction. The assertion should only occur when
triangulation is requested, allowing non-BMesh blender meshes to load
properly.

Note that this fixes a number of .blend regression tests that are
currently baselined as failures.
Merge pull request #385 from jdduke/fix_blend_and_rebase_regression_tests

Fix non-BMesh-containing .blend import