Text gizmos

[ickshonpe]

Objective

Add naive support for text gizmos.

Draw a label at a given point using the default font. 2d only, no rich text support, no anchors, no rotation.

Fixes #16490

Solution

I'm not familiar with the gizmos implementation, so my plan is to hack together something limited that just about works, and then hand it over to a rendering SME or someone to push over the line.

Done

• Added a text_2d method to the Gizmos SystemParam that adds text gizmos for rendering to a GizmoTextBuffer resource.
• In post update, gizmo_text_system does text layout for each text gizmo queued in the GizmoTextBuffer and adds the resulting glyph geometry to the GizmoBuffer.

Todo

• Rendering isn't implemented yet, need to implement some sort of TextGizmoPipeline in bevy_gizmos_render.
• I started off by adding a text_gizmos feature-gate, but it overcomplicated things at this stage, so removed it for now. I'll add it back at the end.

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