iris-gui: scaling fixes and window resize

[iblowmymind]

View > VM screen at 1x measured ~0.9x, and a window resized to ~1x lost rows of pixels. Separately, fullscreen ignored the setting and blew the picture up to a filtered 1.05x, and changing the scale while fullscreen shrank the whole UI into a corner.

• snap_window_to_fb capped the window at 85% of the monitor, silently reducing the requested scale. Reserve only the measured title bar; no guessed menu bar/taskbar allowance either, since 1280x1024 at 1x on a 1080p screen has ~3 points to spare. If the window manager constrains the window anyway, the draw falls back to a filtered fit. Request 1pt of slack so WM rounding can't land just short of 1x.

• is_integer_scale accepted scales within 1% of an integer and drew NEAREST at that slightly-off size, dropping rows. fb_draw_size now snaps a fit that lands just above an integer device scale down to exactly that integer (never up, which would clip), and uses LINEAR otherwise.

• centered_and_justified put the image origin on a fractional device pixel. fb_draw_rect snaps the rect to the device-pixel grid and paint_fb paints it directly so layout doesn't re-round it. Both heads use it.

• The VM screen setting is now the maximum draw scale: a larger window (fullscreen, maximised, manual resize) centres the picture at the chosen scale instead of stretching it; only a smaller window shrinks it to fit.

• snap_window_to_fb doesn't resize a fullscreen window (InnerSize just shrank the rendered area); the request stays pending and is applied once fullscreen ends.

Cross-platform, no new dependencies. Adds unit tests for the draw geometry.