Prefer native BCn in automatic mode for Nvidia too!

[abesmon]

When automatic BCn selection reaches an otherwise unknown driver, use the base driver if it advertises textureCompressionBC and the native policy permits it. The policy defaults on for VK_DRIVER_ID_NVIDIA_PROPRIETARY and can be overridden with WRAPPER_NATIVE_BCN. Existing Qualcomm and Turnip selection, explicit emulation modes, and format-property handling remain unchanged.

Validation

Tested on Nintendo Switch / LineageOS with the switchroot NVIDIA proprietary Vulkan driver.

BCn setting	Native policy	BC1 allocation	BC1/BC3/BC7 byte round-trip	Result
auto	false	64 KiB	transformed by emulation	render passed
auto	unset	32 KiB	exact	render passed
auto	true	32 KiB	exact	render passed
none	false	32 KiB	exact	explicit mode remained authoritative
full	false or true	emulated	transformed	explicit mode remained authoritative