feat: optional host GPU via GPU_MODE=host

[s1oz]

Add an opt-in host GPU path for headless Intel/AMD/NVIDIA boxes. The emulator still defaults to SwiftShader. With GPU_MODE=host and /dev/dri passed through, gfxstream uses Mesa via a small headless modesetting Xorg (GLX) plus an EGL GBM hook, so guest GLES can run on the iGPU without a monitor. Viewing stays on scrcpy / scrcpy-web (-no-window).

No existing compose defaults are changed (port 5555, scrcpy-web, DNS, root/gapps remain as upstream).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update

How Has This Been Tested?

On an Unraid host with Intel UHD 630, GPU_MODE=host, /dev/dri passed through:

• Container glxinfo: Mesa Intel(R) UHD Graphics 630 (CFL GT2), accelerated
• Emulator log: Graphics Adapter Android Emulator OpenGL ES Translator (Mesa Intel(R) UHD Graphics 630 (CFL GT2))
• Guest: adb shell dumpsys SurfaceFlinger | grep GLES → same Mesa Intel string, OpenGL ES 3.0
• ARM64 Unity APK ran via ndk_translation
• Unset GPU_MODE still maps to swiftshader_indirect

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (no unit-test harness; verified on a real iGPU)
• New and existing unit tests pass locally with my changes (N/A)