Fix incorrect workgroupBarrier and OOB array access in auto_exposure

[Kurble]

This commit fixes two issues in auto_exposure.wgsl:

• A storageBarrier() was incorrectly used where a workgroupBarrier() should be used instead;
• Resetting the histogram_shared array would write beyond the 64th index, which is out of bounds.

Solution

The first issue is fixed by using the appropriate workgroupBarrier instead;
The second issue is fixed by adding a range check before setting histogram_shared[local_invocation_index] = 0u.

Testing

These changes were tested using the Xcode metal profiler, and I could not find any noticable change in compute shader performance.