Add separate option to control font size behavior in many_text2d (#17823)

# Objective

I'm working on some PRs involving our font atlases and it would be nice
to be able to test these scenarios separately to better understand the
performance tradeoffs in different situations.

## Solution

Add a `many-font-sizes` option.

The old behavior is still available by running with `--many-glyphs
--many-font-sizes`.

## Testing

`cargo run --example many_text2d --release`
`cargo run --example many_text2d --release -- --many-glyphs`
`cargo run --example many_text2d --release -- --many-font-sizes`
`cargo run --example many_text2d --release -- --many-glyphs
--many-font-sizes`

Author: rparrett

examples/stress_tests/many_text2d.rs

@@ -31,10 +31,14 @@ const CODE_POINT_RANGES: [RangeInclusive<u32>; 5] = [
 #[derive(FromArgs, Resource)]
 /// `many_text2d` stress test
 struct Args {
-    /// whether to use many different glyphs so that multiple separate font atlas textures are used.
+    /// whether to use many different glyphs to increase the amount of font atlas textures used.
     #[argh(switch)]
     many_glyphs: bool,
 
+    /// whether to use many different font sizes to increase the amount of font atlas textures used.
+    #[argh(switch)]
+    many_font_sizes: bool,
+
     /// whether to force the text to recompute every frame by triggering change detection.
     #[argh(switch)]
     recompute: bool,
@@ -186,7 +190,7 @@ fn print_counts(
 }
 
 fn random_text_font(rng: &mut ChaCha8Rng, args: &Args, font: Handle<Font>) -> TextFont {
-    let font_size = if args.many_glyphs {
+    let font_size = if args.many_font_sizes {
         *[10.0, 20.0, 30.0, 40.0, 50.0, 60.0].choose(rng).unwrap()
     } else {
         60.0