Test build: GPU fixes and driver picker (issues #3, #6)

Test build for #3 and #6. Not a real release, just for checking these fixes on the phones that have the problems before they go into the next version.

Installs over v1.2.0 (same signing), saves and game files stay.

What's in it

• GPU driver picker works now (Snapdragon only). On the main screen: Graphics... -> GPU driver... -> Install .zip..., pick an AdrenoTools driver package (for example Turnip from K11MCH1/AdrenoToolsDrivers), select it, Save. The in-game settings show the active GPU at the bottom (GPU: Turnip ... when Turnip is loaded). To undo: pick System driver.
• PowerVR (#3): older PowerVR BXM drivers were rejected by the graphics library, so the game had no Vulkan. They're accepted now (fix from KartPad).
• Budget GPUs (PowerVR GE8320, Mali-T8xx) that only have a "lighter" Vulkan now get Vulkan instead of dropping to slow OpenGL ES.
• Adreno 8xx (#6): stretched graphics fix on by default (from yesterday's commit). Adreno 750 (S24 class): missing-character fix on by default. Neither is confirmed on those phones yet.
• Launch crash on some older Adreno drivers removed (from KartPad).
• If Vulkan can't start, the game now logs why. Export Diagnostics on the main screen includes it.
• Also in this build: the settings menu scrolls by dragging, a notice when the game had to switch to OpenGL ES, resolutions below 1x removed (they crashed), and it runs cooler (shader rebuild throttled, 60 Hz display, profile-optimized build).

Note: the first launch rebuilds the shader cache, so expect some stutter for the first few minutes.