fix(hud): offscreen legacy HUD rendering across 1.21

[saadndm]

pls merge #979 first

Description

• Changed TextureTarget to its superclass RenderTarget, since no TextureTarget specific stuff is used it is the same functionally, and 1.8 only has RenderTarget

DebugOverlayOffscreen.kt:

• No funtional changes, except fixing f3 rendering to wrong target only on 1.21.4 because Minecraft's GUI flush rebinds the main framebuffer in renderImmediateLegacy
• Moved shared stuff into SkiaOffScreenTarget
• Removed unused invalidate() function

OneConfig.java:

• Call LegacyHudOffScreen.render() on all versions

LegacyHudOffscreen.kt:

• Uses shared SkiaOffscreenTarget
• Copies how DebugOverlayOffscreen handles version specific rendering paths
• Cleares stale depth on 1.21.5 and 1.21.8 to so that legacyhud does not disappear when OneConfig open
• Removed unused invalidate() function

Testing

Tested that legacyhud items now render above blur on 1.21.x version
Tested that f3 now renders on top in 1.21.4

Checklist

• I made a clear description of what was changed
• I stated why these changes were necessary
• I updated documentation or said what needs to be updated
• I made sure these changes are backwards compatible
• This pull request is for one feature/bug fix