UEFI recommends y=.382 (1 - 1 / (golden ratio)), make this the default.
Fractional position starts with a decimal point: x=.5 y=.382
Pixel offset relative to the center is set as before: x=123 y=456
If the coordinates were omitted, old default was: x=0 y=0
Relative coordinates make it easier to center the image.
The value "auto" is now replaced with simply 0.
The value "native" is replaced with "keep" for consistency.
The value is clamped, so x=-9999 will align to the left border etc.
