text: centralize SoText2 raster preparation

[tritao]

Purpose

Create the shared private text-preparation seam used by both renderers.

Commit 1: shared text preparation

• centralize multiline SoText2 layout, UTF-8 traversal, justification, glyph positioning, and coverage generation;
• keep glyph bitmap data as non-owning pointers into the existing glyph cache;
• prepare raster data only after the normal LegacyGL visibility/culling decision;
• preserve the historical out-of-bounds glyph warning;
• add a native multiline LEFT/CENTER/RIGHT layout test;
• add the LegacyGL text2_layout_basic visual baseline.

The LegacyGL path consumes this result with its existing glBitmap and glDrawPixels behavior. The retained path consumes the same result in the dependent raster/text implementation without duplicating layout.

Commit 2: GL gating split

Separate common shape gating and late GL setup from primitive-only triangle sorting:

• preserve the original shouldGLRender() ordering;
• let direct-rendered SoText2 use begin() and finish() while skipping only triangle sorting;
• keep the focused direct-raster transparency and shape-gating regression tests.

Scope

This is a private LegacyGL-preserving text-preparation foundation. It adds no public planner/API, transparency ordering policy, or retained execution policy.