USD importer: fixed typo

Fixed typo in copyright block comments.

USD importer: updated copyright notices.

Explicitly acknowledged adaptation from the Alembic
importer implementation and copyright by Kevin Dietrich.

USD import: remove placeholder bezier curve code

The placeholder code for the unimplemented functionality
of importing bezier curves was causing a crash. It has
been removed until support for beziers can be properly
added.

Updated USDMeshReader comments

USD import: update copyright notices

USD import: convert sun angle to radians

Now converting USD distant light angle from degrees
to radians.

USD import: validate light attributes

Added checks to validate light attributes
and values, to help avoid errors when reading
light settings which might not have been
authored.

USD import: bug detecting mesh topology change

Fixed mesh reader logic detecting topology changes in mesh
sequence animations, which avoids a crash in some cases.

USD import: gray out disabled options.

Now graying out, instead of hiding, disabled options.

USD import options Experimental header alignment

Fixed alignment of the Experimental column header
to be consistent with the other options headers.

USD import: don't check for cache usdz extension.

Removed check for .usdz extension in logic to
determine cachfile type.

USD Import: removed unused variable.

USD import: use pointers for out parameters.

Changed return parameter types from references to
pointers, to improve code readability, per suggestion
from Sybren.

USD import: get_opacity_threshold() function.

Moved the opacity threshold query logic out of the
needs_blend() function to a new get_opacity_threshold()
function, for readability.

USD import: fix comments.

Updated to C-style comments. Minor cleanup of
comments text.

USD import: update copyright notice.

Added NVIDIA copyright notice.

USD import cleanup: reorder include.

USD Import: fix comments.

Now using C-style comments throughout. Also,
putting unused code in #if 0 blocks. Removed
unneeded comments and other miscellaneous fixes.

USD Import: only create UV data if needed.

Fixed bug where UV custom data was being created
even if reading mesh UVs is disabled.

USD Import: read uvs and colors options.

Replaced the mesh_read_flag enum property with
read_mesh_uvs and read_mesh_colors toggles, as
the enum property has turned out to be confusing
to users.