fix(rendering): use the default clear color for window letterbox margins

[joeykchen]

Clear uncovered window regions with the existing default clear color in both GLES3 and RenderingDevice compositors, while keeping margins opaque. Projects can control their margins through default_clear_color or RenderingServer.set_default_clear_color without a new API or changes to viewport layout and input coordinates.

Base: spx4.4.1. This is the engine-side prerequisite for the SPX stage-letterbox branch.

Prior build and runtime validation: Matching Native and Web builds passed. Eight macOS OpenGL3/Metal window scenarios checked white and runtime color changes, horizontal/vertical margins and transparent input colors; Web DPR 1/2 resize samples stayed opaque white with unchanged visible-world bounds.

[fennoai]

Review: Use project background color for uncovered window regions

This is a small, well-scoped fix. Both call sites replace the hardcoded black clear color with texture_storage->get_default_clear_color() (alpha forced to 1.0) so letterbox/uncovered window regions match the project's background color instead of always being black.

No issues found. The change is correct, safe, and consistent with existing usage.

• Correctness & consistency — The new usage matches how get_default_clear_color() is already used for related purposes elsewhere (window border/letterbox fill in display_server_windows.cpp, early-window clear in display_server.cpp, viewport background in renderer_viewport.cpp). Both paths correctly force alpha to 1.0 to keep regions opaque. The RD call is placed once outside the per-target loop; the GLES3 call stays guarded behind the p_first + letterbox-region check.
• Performance — get_default_clear_color() is a trivial accessor returning a cached Color field (no allocation, no lock, no settings lookup), so per-frame cost is negligible.
• Thread safety — Reading the cached clear color on the render thread follows the established pattern already used by several per-frame render-thread call sites; no new concern is introduced. The existing comment about avoiding a DisplayServer window-size query (deadlock in separate-render-thread mode) still holds.
• Memory safety — texture_storage is created in each constructor and already dereferenced unconditionally in both functions, so the added call introduces no new null-dereference risk.
• Comments — Both updated/added comments accurately describe the code.

Optional (testing note, not a blocker): get_default_clear_color() returns the raw rendering/environment/defaults/default_clear_color value and is passed directly without an sRGB↔linear conversion — matching the sibling usages. Worth a quick visual confirmation that the letterbox fill matches the actual rendered viewport background across both renderers (and in HDR mode), since scene-render paths may apply the color in linear/sRGB framebuffer contexts. This is an existing engine-wide consideration, not introduced by this PR.