Fix pressure (#3877)

This PR fixes pen pressure. Previously, `toFixed` loses the `z` of the
point.

### Change Type

- [x] `sdk` — Changes the tldraw SDK
- [x] `bugfix` — Bug fix

### Test Plan

1. use a pen with pressure

### Release Notes

- Fixes pen pressure.

packages/tldraw/src/lib/shapes/draw/toolStates/Drawing.ts

@@ -177,7 +177,8 @@ export class Drawing extends StateNode {
 		// have our own value for this. The inputs.isPen is only if the input is a regular
 		// pen, like an iPad pen, which needs to trigger "pen mode" in order to avoid
 		// accidental palm touches. We don't have to worry about that with styluses though.
-		const z = this.info.point.z === undefined ? 0.5 : this.info.point.z
+		const { z = 0.5 } = this.info.point
+
 		this.isPen = isPen
 		this.isPenOrStylus = isPen || (z > 0 && z < 0.5) || (z > 0.5 && z < 1)
 
@@ -304,8 +305,8 @@ export class Drawing extends StateNode {
 		const { segments } = shape.props
 
 		const { x, y, z } = this.editor.getPointInShapeSpace(shape, inputs.currentPagePoint).toFixed()
-
-		const newPoint = { x, y, z: this.isPenOrStylus ? +(z! * 1.25).toFixed(2) : 0.5 }
+		const pressure = this.isPenOrStylus ? +(inputs.currentPagePoint.z! * 1.25).toFixed(2) : 0.5
+		const newPoint = { x, y, z: pressure }
 
 		switch (this.segmentMode) {
 			case 'starting_straight': {