v6.338.1

This release fixes a number of minor issues with v6.338.0, as well as a
major regression of the frame mixing code.

Fixes and performance enhancements:

• fix H.274 grain database PRNG
• fix compilation on FreeBSD
• fix unnatural peak detection flicker on mostly black samples
• fix plplay 'Use detected luminance' checkbox
• fix unnecessary recompilation when changing antiring strength
• properly disable anti-ringing for non-negative filters
• fix double-callback on sliced texture uploads
• dispatch callback slightly sooner in pointer texture upload fallback
• slightly improve saturation when tone mapping by large factors
• various improvements to the benchmark
• fix regression when frame mixing low FPS sources
• update various misleading comments