Fix legacy sprite texts not matching stable with respect to fixed width

stable's `pSpriteText` has a `TextConstantSpacing` flag, that is
selectively enabled for some usages. In particular, these are:

- mania combo counter (not yet implemented)
- taiko combo counter (not yet implemented)
- score counter
- accuracy counter
- scoreboard entries (not yet implemented)

Everything else uses non-fixed-width fonts.

Hilariously, `LegacySpinner` _tried_ to account for this by changing
`Font` to have `fixedWidth: false` specified, only to fail to notice
that `LegacySpriteText` changes `Font` in its BDL, making the property
set do precisely nothing. For this reason, attempting to set `Font`
on a `LegacySpriteText` will now throw.

osu.Game.Rulesets.Osu/Skinning/Legacy/LegacySpinner.cs

@@ -87,7 +87,7 @@ private void load(DrawableHitObject drawableHitObject, ISkinSource source)
                         Origin = Anchor.Centre,
                         Scale = new Vector2(SPRITE_SCALE),
                         Y = SPINNER_TOP_OFFSET + 299,
-                    }.With(s => s.Font = s.Font.With(fixedWidth: false)),
+                    },
                     spmBackground = new Sprite
                     {
                         Anchor = Anchor.TopCentre,
@@ -102,7 +102,7 @@ private void load(DrawableHitObject drawableHitObject, ISkinSource source)
                         Origin = Anchor.TopRight,
                         Scale = new Vector2(SPRITE_SCALE * 0.9f),
                         Position = new Vector2(80, 448 + spm_hide_offset),
-                    }.With(s => s.Font = s.Font.With(fixedWidth: false)),
+                    },
                 }
             });
         }

osu.Game/Skinning/LegacyAccuracyCounter.cs

@@ -25,6 +25,7 @@ public LegacyAccuracyCounter()
         {
             Anchor = Anchor.TopRight,
             Origin = Anchor.TopRight,
+            FixedWidth = true,
         };
     }
 }

osu.Game/Skinning/LegacyScoreCounter.cs

@@ -28,6 +28,7 @@ public LegacyScoreCounter()
         {
             Anchor = Anchor.TopRight,
             Origin = Anchor.TopRight,
+            FixedWidth = true,
         };
     }
 }

osu.Game/Skinning/LegacySpriteText.cs

@@ -1,6 +1,7 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+using System;
 using System.Threading.Tasks;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics.Sprites;
@@ -13,6 +14,7 @@ namespace osu.Game.Skinning
     public sealed partial class LegacySpriteText : OsuSpriteText
     {
         public Vector2? MaxSizePerGlyph { get; init; }
+        public bool FixedWidth { get; init; }
 
         private readonly LegacyFont font;
 
@@ -22,6 +24,15 @@ public sealed partial class LegacySpriteText : OsuSpriteText
 
         protected override char[] FixedWidthExcludeCharacters => new[] { ',', '.', '%', 'x' };
 
+        // ReSharper disable once UnusedMember.Global
+        // being unused is the point here
+        public new FontUsage Font
+        {
+            get => base.Font;
+            set => throw new InvalidOperationException(@"Attempting to use this setter will not work correctly. "
+                                                       + $@"Use specific init-only properties exposed by {nameof(LegacySpriteText)} instead.");
+        }
+
         public LegacySpriteText(LegacyFont font)
         {
             this.font = font;
@@ -33,7 +44,7 @@ public LegacySpriteText(LegacyFont font)
         [BackgroundDependencyLoader]
         private void load(ISkinSource skin)
         {
-            Font = new FontUsage(skin.GetFontPrefix(font), 1, fixedWidth: true);
+            base.Font = new FontUsage(skin.GetFontPrefix(font), 1, fixedWidth: FixedWidth);
             Spacing = new Vector2(-skin.GetFontOverlap(font), 0);
 
             glyphStore = new LegacyGlyphStore(skin, MaxSizePerGlyph);