macOS: firstRect should return full rect width/height

Fixes #2473

This commit changes `ghostty_surface_ime_point` to return a full rect
with the width/height calculated for the preedit.

The `firstRect` function, which calls `ghostty_surface_ime_point` was
previously setting the width/height to zero. macOS didn't like this. We
then changed it to just hardcode it to width/height of one cell. This
worked but made it so the IME cursor didn't follow the preedit.

src/apprt/structs.zig

@@ -24,6 +24,8 @@ pub const CursorPos = struct {
 pub const IMEPos = struct {
     x: f64,
     y: f64,
+    width: f64,
+    height: f64,
 };
 
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