Log Message:
libplacebo: update to 6.338.2.

v6.338.2

This release fixes a number of minor issues with v6.338.1, in particular
a fix for the new caching subsystem when using custom cache callbacks.

Fixes and performance improvements:
- properly align AVFrame planes when using oddly-sized pixels
- correct some reported GPU buffer size limits
- properly report texture transfer failures
- avoid ICC-related error spam
- avoid using some deprecated APIs and syntax
- fix pl_find_filter_* missing symbols
- fix duplicate initialization of vulkan debug layers
- fix assertion when using custom cache allocator API
- avoid redundant scale shader variable declarations
- avoid redundant linearization step when using sigmoid scaling
- avoid generating redundant dependency on xxhash
- fix possible integer overflow on 32-bit textures
- fix cropped frame dimensions when using vulkan hwframes