view: account for base size in resize indicator

view: account for base size in resize indicator

For views with a non-pixel size increment (e.g. X11 terminals), it's
helpful to subtract the base size of the window (typically including
menu bar, scrollbars, etc.) before computing the number of size
increments (e.g. cells/characters). This way, the displayed size will
exactly match the terminal grid (e.g. 80x25 or whatever).

wlr_box isn't really the best fit for size hints, so let's define a
struct view_size_hints and a nice view_get_size_hints() function,
wrapping view->impl->get_size_hints().

This also seems like a great opportunity to make view_adjust_size()
window-system-agnostic and eliminate xwayland_apply_size_hints().