videodecoder: Make sure to clear "too slow decoder" if we switch decoder

Fixes #2032
Set page in loading state during video demuxer init

Fixes #2015
rpi: Show core temperature in sysinfo

Fixes #2033
httpclient: Correctly deal with HEAD req. returning 404

Fixes #1927
osx: Add support for zero copy video rendering with VDA
metadata: Split metadata.c into multiple files
backend/image: Fix broken locking around image loader