rendering: add core OpenGL DrawList backend

[tritao]

Summary

Add the OpenGL 3.3 DrawList executor using GLSL 330 core semantics. The backend supports both OpenGL compatibility and core contexts.

• Introduce the renderer backend interface.
• Add DrawList-specific OpenGL dispatch.
• Execute retained geometry through SoGLRenderBackend.
• Keep VBO, index-buffer, texture, program, and VAO ownership inside the backend.
• Establish a deterministic frame baseline: viewport, matrices, clear color/depth, and required baseline GL state.
• Render retained positions, vertex colors, indices, and the basic visual shader path.
• Preserve luminance and luminance-alpha texture upload adaptation needed by the visual path.
• Generate composable GLSL roots with recursive relative includes and exact build depfiles.
• Validate the executor on OpenGL 3.3 compatibility and core contexts.
• Integrate CoinVisualTests with base unlit rendering, model transforms, vertex colors, orthographic projection, and depth/occlusion parity scenes.

This layer deliberately does not own retained material semantics, texture policy,
depth-function/write policy, blending, raster semantics, picking, selection,
render stages, or manager orchestration. Those belong to later stack layers.

Visual validation

CoinVisualTests can be built with or without LegacyGL. LegacyGL is the sole
authority for checked-in reference baselines; DrawList compatibility and core
runs compare against the same LegacyGL images. Baseline updates are rejected
for DrawList and are unavailable when LegacyGL is not built.

The base visual coverage includes:

• unlit_cube_basic
• unlit_transformed_cubes
• unlit_vertex_colors
• unlit_orthographic_basic
• unlit_depth_overlap

The DrawList visual adapter rejects a frame when commands contain inconsistent
view/projection matrices, because this layer executes one frame-level matrix
pair. Material, lighting, blending, raster, picking, manager, and lifecycle
coverage remains with its owning layer.

The retained-IR test also verifies that indexed per-vertex material colors are
captured into backend-neutral geometry data, including effective alpha.

Build-system requirement

CMake 3.21 or newer is required for the GLSL root-shader embedding pipeline,
which uses build-time dependency/depfile support. This is a build-system
requirement; DrawList rendering itself does not intrinsically require CMake
3.21.

Stack position

This is the base DrawList execution layer, built on the retained render-IR
layer. Later layers add material and lighting semantics, text preparation,
raster and image state, picking, planning, manager orchestration, and staged
rendering/lifecycle behavior.