Modernize the graphics pipeline: legacy OpenGL -> Qt RHI

[aalex]

Summary

Migrate MapMap's rendering from legacy (fixed‑function) OpenGL to a modern graphics API — Core‑profile OpenGL and/or Metal. Epic / long‑term.

Why

• MapMap renders with legacy OpenGL (~2.1: glBegin/glEnd, glVertex, glTexCoord, fixed‑function), used throughout (ShapeGraphicsItem, the GL canvases, texture handling).
• Apple deprecated all of OpenGL in 2018; on Apple Silicon there is no native OpenGL driver — it is emulated on Metal (AppleMetalOpenGLRenderer). The legacy fixed‑function path is the most fragile part of that emulation.
• Concrete symptoms today: Syphon output fails (Syphon output (server) unsupported on legacy OpenGL / Apple Silicon #620 — IOSurface texture creation in the legacy context floods cannot create texture, Metal texture cache was released), and we depend on a deprecated, emulated path that a future macOS could remove.

Scope (large)

• Replace fixed‑function drawing with shaders + vertex buffers (Core profile), or a Metal/abstraction backend.
• Touches the whole render chain: canvases, shape/texture graphics items, texture upload & sharing.
• Cross‑platform constraint: MapMap also targets Linux/Windows, where OpenGL is the native standard. Core‑profile GL keeps a single codebase; Metal is macOS‑only and would need an abstraction (e.g. Qt RHI, or Vulkan + MoltenVK).

Benefits

• Long‑term viability on macOS (no longer relying on emulated, deprecated OpenGL).
• Unblocks Syphon output (Syphon output (server) unsupported on legacy OpenGL / Apple Silicon #620).
• Likely performance gains and access to modern GPU features.

Options to evaluate

1. Core‑profile OpenGL (3.2+) — modern GL, still cross‑platform; best effort/reward ratio.
2. Qt RHI (Qt 6's rendering abstraction over OpenGL/Vulkan/Metal/D3D) — most future‑proof + cross‑platform, larger migration.
3. Metal (+ Syphon Metal server) — most durable on Mac, but macOS‑only.

Related: #620 (Syphon output).

[aalex]

Decision: target Qt RHI as the modern rendering backend.

Rationale: MapMap must keep Linux and Windows, so a macOS‑only path (pure Metal) is out. Qt RHI is Qt 6's rendering abstraction over Metal / Vulkan / OpenGL / Direct3D — one codebase, native backend per platform (Metal on macOS), and it gets us off the deprecated/emulated OpenGL path on Apple Silicon.

Implications

• Rewrite the fixed‑function rendering (glBegin/glEnd, glVertex/glTexCoord) into RHI: shaders (baked with qsb), vertex/uniform buffers, pipeline state objects.
• Move the render surface from QGraphicsView + QOpenGLWidget (beginNativePainting) to QRhiWidget — requires Qt ≥ 6.7, which raises the project's minimum Qt version.
• RHI is a semi‑private API (QT += gui-private), still stabilizing → expect some maintenance churn across Qt versions.

Unblocks #620 (Syphon output): on macOS, RHI renders with Metal, and QRhiTexture::nativeTexture() exposes the MTLTexture to publish via Syphon's Metal server (SyphonMetalServer).

Optional intermediate: a Core‑profile OpenGL step could unblock Syphon sooner while staying cross‑platform, but targeting RHI directly avoids doing the migration twice.