Support mix of absolute and relative values for geometry

Use case

I use mpv for language learning - I always open it alongside vocabsieve (used for sentence mining). I always put vocabsieve on the right of my screen and it always has a fixed width of 365 pixels. I want mpv to take up the remaining space on the screen (see screenshot). Hence I would like an option to have something like geometry=100%-365x100% (use 100% width but subtract 365px).

I also use mpv on multiple PCs with the same config, so at the moment this is the only config line that is different between them (as I need to specify the absolute width manually for both of them)

Expected behavior of the wanted feature

I don't really mind how the params will be provided, but the geometry option is already quite wordy when specifying both the starting position and window's dimension. So in order to distinguish from window offset, brackets may need to be introduced. E.g. something like geometry=(100%-365)x100%[+-x+-y]

Start by tracing how the geometry option is parsed and documented; the issue provides the current percentage form and the proposed mixed expression (100%-365)x100%[+-x+-y]. Define and implement the accepted syntax, including how absolute and relative values interact, then verify that geometry can reserve a fixed 365-pixel side area across screen sizes.