Mesh: add thin instances

[Popov72]

I took model on the regular instance implementation for the thin instance implementation:

• added a property _thinInstanceDataStorage of type _ThinInstanceDataStorage to hold the main data (much like _instanceDataStorage / _InstanceDataStorage).
• created an additional _userThinInstanceBuffersStorage to hold data for additional custom attributes other than the matrices (much like _userInstancedBuffersStorage). This way, if no custom attributes are added, no penalty paid

I have prefixed all methods by thinInstance, I can change that (notably, addThinInstance is thinInstanceAdd instead).

By default, the bounding box is recomputed to encompass all thin instances, but it can be disabled by setting doNotSyncBoundingInfo to true (already existing property).

As discussed, thin instances are always displayed with shader code assuming non uniform scaling, but with an optimized code (shearing not supported).

[Popov72]

Hum, shouldn't I add an optional parameter to the thinInstanceSetBuffer method to create a static buffer instead of a dynamic one, in case the user know they won't update the buffer?

[sebavan]

sounds totally fine to me :-)

[sebavan]

Also should try to ensure we use the types that are not creating dupplicate in the engine updateBuffer ???

[deltakosh]

ALLL GOOD. You rock

Can you add a visual test for thinInstances?

[Popov72]

I also wonder if we shouldn't also let the user be able to set the current number of instances (but always < matrixBuffer.length / 16): that way, they would be able to allocate a bigger buffer at start in case they need to have more instances during the course of the program, they would not have to reallocate / copy a buffer each time they have to add an instance.

[Popov72]

Visual test added