fix: update FFmpeg on Linux to avoid GStreamer ABI conflict

[BenJule]

Summary

Update the bundled FFmpeg dependency from 7.0.2 to 7.1.5 on Linux to avoid an ABI conflict with newer system GStreamer libav plugins.

macOS remains on FFmpeg 7.0.2 and the MSVC/Windows dependency is unchanged.

Fixes #12074

Problem

The generated Linux launcher exposes BambuStudio's bundled libraries through:

export LD_LIBRARY_PATH="$DIR/bin"

BambuStudio currently bundles FFmpeg 7.0.2, including:

libavcodec.so.61
libavutil.so.59
libswresample.so.5
libswscale.so.8

On Debian 13, the system GStreamer libav plugin is built against FFmpeg 7.1.5.

Both FFmpeg versions expose the same libavutil.so.59 SONAME, but the bundled FFmpeg 7.0.2 library does not provide:

av_mastering_display_metadata_alloc_size

The system GStreamer plugin requires that symbol.

This causes gst-plugin-scanner to fail with:

libavformat.so.61:
undefined symbol: av_mastering_display_metadata_alloc_size,
version LIBAVUTIL_59

Fix

Use FFmpeg 7.1.5 for Linux builds while keeping the existing FFmpeg 7.0.2 dependency for macOS.

The MSVC dependency is not modified.

The Linux FFmpeg source archive is updated to:

n7.1.5
SHA256=e3963a50831c985933e1a625ed566ec4c7adb5c012c34fa9f84438e1d61bdacc

FFmpeg 7.1.5 retains the same relevant major library versions while providing the symbol required by newer GStreamer builds.

Validation

Before changing the dependency, the issue was reproduced using the packaged Linux build with the normal BambuStudio launcher.

With bundled FFmpeg 7.0.2:

gst-plugin-scanner:
Failed to load plugin ... libgstlibav.so

undefined symbol:
av_mastering_display_metadata_alloc_size
version LIBAVUTIL_59

As an A/B runtime test, the FFmpeg 7.0.2 libraries in an isolated copy of the package were replaced with the installed Debian 13 FFmpeg 7.1.5 libraries while keeping the original BambuStudio launcher and its LD_LIBRARY_PATH.

Results:

SYSTEM_LIBRARIES=OK
REPLACEMENT=OK
LIBRARIES=OK
av_mastering_display_metadata_alloc_size@@LIBAVUTIL_59
GSTREAMER_FFMPEG_FIX=PASS

BambuStudio started normally through the original launcher and the GStreamer/FFmpeg undefined-symbol warning disappeared.

Existing unrelated GTK warnings such as:

gtk_window_resize: assertion 'width > 0' failed

are unaffected and are not part of this fix.

Scope

Only:

deps/FFMPEG/FFMPEG.cmake

is changed.

The actual dependency change still needs to be validated by CI by building the Linux package with FFmpeg 7.1.5 and then testing that generated artifact through the normal launcher.