WebKitGTK's accelerated GL compositor crashes with a SIGSEGV inside
g_application_run on some Intel setups, hitting Mesa anv/i965 code paths
for DRM format modifiers that aren't implemented (FINISHME: YUV
colorspace / multi-planar formats). Disabling the DMA-BUF renderer alone
doesn't cover the GL compositor, so the crash survived that workaround.
Set WEBKIT_DISABLE_COMPOSITING_MODE=1 in init() (skipped if the user
already set it) to force CPU rendering, which is fine for a UI this
small and sidesteps the broken modifier path.
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