fix: Collada replace <polylist> export by <triangle> export for triangulated meshes.

This is a regression from Blender 2.79 where the usage
of <triangles> was already implemented, but unintentionally
removed in Blender 2.80

Also renamed variables for better reading.

fix T60049: Collada Exporter: When exporting multiple UVs, the set index must start with 0

fix T59743: Collada exporter: Add option for exporting flat curves

The Collada exporter suppresses the export of flat animation curves
to optimize the animation (in fact to make the exported file smaller).

But sometimes it is important to also have the flat curves exported
because they may be needed to define an initial transformation to
a fixed location - like translating the weapon from the ground floor
to the back of the model in the report.

I added a new option "all keyed curves" which is disabled by default
but when enabled it also exports flat curves.

feedback is very welcome

fix T60092 collada exporter: exporting a rig with one single bone fails

The problem was that removing entries from a vetor while iterating
the vector was implemented badly. This caused the failure when only
one element was in vector.

Merge remote-tracking branch 'origin/master' into blender2.8

Fix (devtalk 4053): Collada build with older cmake versions broken

Merge remote-tracking branch 'origin/master' into blender2.8

fix: Collada: remove Class from method definition in header file

Merge remote-tracking branch 'origin/master' into blender2.8

fix T58568: used wrong case in variable name