Add HDR support for non-metal backends

[kunaltyagi]

nanogui/src/common.cpp

Lines 210 to 216 in 2ee903c

	std::pair<bool, bool> test_10bit_edr_support() {
	#if defined(NANOGUI_USE_METAL)
	return metal_10bit_edr_support();
	#else
	return { false, false };
	#endif
	}

This hardcodes the HDR support for only metal backend. Windows and Linux display managers have decent HDR support and effort could be made to support them as well.