Add ffmpeg H.264/MKV video recording with configurable format selection

[Copilot]

The SHIFT-M recording feature used a hand-rolled FLC encoder producing 540MB files for typical gameplay. This adds H.264/MKV encoding via the existing ffmpeg integration, reducing file sizes ~50-100x while maintaining playability in standard media players.

Changes

MKV Encoder (src/bflib_fmvids.cpp)

• H.264 encoder with palette conversion pipeline: 8-bit indexed → RGB24 → YUV420P
• CRF 23, GOP 114 frames (~2s at 57 FPS), medium preset
• Outputs to scrshots/game####.mkv

Configuration

• Added VIDEO_RECORDING_FORMAT to config_keeperfx.c supporting MKV (default) and FLC
• Format constants defined in scrcapt.h (VIDEO_FORMAT_MKV, VIDEO_FORMAT_FLC)

Integration (src/scrcapt.c)

• Recording functions dispatch to MKV or FLC encoder based on configuration
• Original FLC implementation preserved unchanged

Build

• Added libswscale to deps/CMakeLists.txt for pixel format conversion

Example Configuration

; File format for video recording (SHIFT-M); MKV or FLC.
; MKV provides 50-100x better compression and is playable in standard media players.
VIDEO_RECORDING_FORMAT=MKV

Technical Notes

• Maintains 57 FPS framerate matching original FLC format
• Requires 8-bit color mode (existing constraint)
• MKV encoder allocates two frames (RGB24 intermediate, YUV420P output) plus sws_context
• Uses existing ffmpeg dependencies, no new external libraries

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