Tracking: Re-write marker request function

There are two main things.

First, remove the marker index caching. Thins makes it possible to
safely use function from a threaded environment.

Second, replace linear search with binary search, which speeds up
random lookup.

There is no measurable difference in the stabilization which had a
comment about caching nature of track lookup. The random lookup
complexity changed from O(N) to O(log N). In practice this also
unlikely to be measurable, but thread-safety worth it.

Tracking: Add interpolated accessor for markers

Allows to get marker position interpolated through out a gap.

Currently unused, but is covered with test. Aimed to be used for
track average operator.

Tracking: Implement tracks average operator

Average selected tracks into the new one. This can be used to improve
stability of tracking on blurry or non-very-sharp feature shapes.

Averaging happens for all position, pattern corners and search area.
Disabled markers do not take effect on averaging. Keyframed flag is
copied from source.

Gaps in the source tracks will be linearly interpolated, to reduce
result track jump. Note that this only applies to gaps "inbetween".
This means that if an input track doesn't have markers in the
beginning/end of it, there is nothing to interpolate with and the
result track will jump.

Available from the Track panel, under the Merge category.

Differential Revision: https://developer.blender.org/D6323

Merge branch 'blender-v2.92-release'

Cycles: Fix missing OpenCL extensions in certain cases

If extensions string is longer than 1024 then the old code would have
reported empty string instead of extensions.

Now the code does dynamic string allocation to store result of request,
similar to what is done in `OpenCLInfo::get_hardware_id`.

The code looks a bit ugly, but it didn't really change much with this
patch. In other words, the code can become more modern and clear, but
it is considered to be outside of the scope of this change.

Differential Revision: https://developer.blender.org/D10135

Tracking: Fix transform cancel for plane tracks

Use exact marker accessor. Harmless due to the current way the
transform system is used by tracking. But for the future development
proper accessor needs to be used.

Merge branch 'blender-v2.92-release'

Fix T84167: Saving half-float EXR might result in NaN pixels

Clamp value to the -HALF_MAX .. HALF_MAX.

The non-clamped values were causing NaN and inf values saved to
the file, which was the root cause of glare node giving unexpected
result.

The nan/inf on overflow is something mentioned in the half data
type in OpenEXR header.

Differential Revision: https://developer.blender.org/D10105

Tracking: Simplify transform code

Reduce amount of duplicated pointer offset logic: advance transform
data pointers deep in the loop rather than have offset duplicated in
the outer loop.

Array size calculation still has duplicated, but that is another story.

Tracking: Re-duplicate logic used to count and initialize data

Allows to easily implement more comprehensive checks about which
markers get added to the transformation context.

No user measurable changes are expected, purely house-keeping to
ease an upcoming development.