Preserve the whitespaces when getting text from FreeText annotations (bug 1871353)

When the text of an annotation is extracted in using getTextContent, consecutive white spaces
are just replaced by one space and. So this patch add an option to make sure that white
spaces are preserved when appearance is parsed.
For the case where there's no appearance, we can have a fast path to get the correct string
from the Content entry.
When an existing FreeText is edited, space (0x20) are replaced by non-breakable (0xa0) ones
to make to see all of them on screen.

src/core/default_appearance.js

@@ -390,6 +390,26 @@ endcmap CMapName currentdict /CMap defineresource pop end end`;
     return this.resources;
   }
 
+  static getFirstPositionInfo(rect, rotation, fontSize) {
+    // Get the position of the first char in the rect.
+    const [x1, y1, x2, y2] = rect;
+    let w = x2 - x1;
+    let h = y2 - y1;
+
+    if (rotation % 180 !== 0) {
+      [w, h] = [h, w];
+    }
+    const lineHeight = LINE_FACTOR * fontSize;
+    const lineDescent = LINE_DESCENT_FACTOR * fontSize;
+
+    return {
+      coords: [0, h + lineDescent - lineHeight],
+      bbox: [0, 0, w, h],
+      matrix:
+        rotation !== 0 ? getRotationMatrix(rotation, h, lineHeight) : undefined,
+    };
+  }
+
   createAppearance(text, rect, rotation, fontSize, bgColor, strokeAlpha) {
     const ctx = this._createContext();
     const lines = [];