Draw: pen pressure scales ink width

A pen reading of 1 is the full pen width. A lighter touch draws a thinner
stroke, and a zero reading stays visible at 8% of the pen. A mouse, a
missing or non-finite reading, and a stroke whose samples are all the old
placeholder 0.5 still draw at the pen width. PPTX keeps one width per
stroke: the mean of the samples for a pressured stroke, the pen width for
a legacy one. Tilt is not read.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/control-protocol.md

@@ -17,7 +17,7 @@ Coordinates are slide points (1/72 inch; a 16:9 slide is 960 × 540) unless note
 | `document.inspect` | — | Slides, sections, layouts, shapes, selection |
 | `ui.inspect` | — | UI state, active tool, window/canvas/slide rects, dialog, `home` (the File tab open over a presentation), `backstage` (its page, also the start screen), slide show state, documents, perf (fps, render ms) |
 | `ui.tool.select` | `{tool, preset?}` | `select`, `text`, `shape` + preset, any preset name, `curve`, `freeform`, `scribble`, `pen`, `highlighter`, `eraser` |
-| `ui.pointer` | `{events: [{kind: down\|drag\|up\|move\|doubleclick\|tripleclick, x, y, mods?}], mods?}` | Drive the active tool in slide points through the same code path as the mouse |
+| `ui.pointer` | `{events: [{kind: down\|drag\|up\|move\|doubleclick\|tripleclick, x, y, mods?, pressure?}], mods?}` | Drive the active tool in slide points through the same code path as the mouse. `pressure` is `0..=1` for a pen; omit it for a mouse |
 | `ui.key` / `ui.text` | `{key, shift?, alt?, cmd?}` / `{text}` | Keyboard input (also drives a running slide show) |
 | `ui.ime` | `{preedit, active?: [start, end]}` / `{commit}` | An input method's composition through egui, as a real one arrives: shown in place on the slide or in a focused field; `{"preedit": ""}` cancels |
 | `ui.move` / `ui.click` / `ui.drag` | screen points | Real egui pointer input — reaches every widget, menu and pane |