text: centralize SoText2 raster preparation

[tritao]

Summary

This prerequisite centralizes SoText2 screen-space layout and glyph coverage generation behind the node’s private implementation.

LegacyGL and retained rendering now consume the same prepared raster result:

• shared glyph layout, UTF-8 traversal, positioning, justification, and coverage generation
• separate binary and antialiased glyph coverage with independent raster origins
• LegacyGL keeps its existing glBitmap and glDrawPixels execution behavior
• the retained path adapts the shared result into pixel-raster commands without reimplementing text layout

The second commit separates common shape GL gating from primitive-only triangle sorting while preserving the historical statement order:

• SoText2 skips only sorted-triangle handling
• inherited transparency and depth-write behavior remain intact
• post-sort special paths remain in their original order
• focused LegacyGL regressions cover direct-rendered transparency and gating order

Scope

This is a LegacyGL-preserving prerequisite. It contains no retained-renderer API, shader, or render-ordering redesign. It establishes the reusable text-preparation seam consumed by #50 while keeping direct LegacyGL behavior explicit and independently testable.

Commits

1. text: centralize SoText2 raster preparation
2. rendering: separate primitive triangle handling from shape GL gating

Stack

Layer 14/18, based on PR #49.