windows: add opt-in D3D11 shader support

[GGRei]

Summary

Adds opt-in D3D11 support for v-gui’s built-in shader pipelines.

This PR moves the built-in GUI shaders to a generated v shader pipeline with OpenGL, Metal, and HLSL/D3D11 output, while keeping D3D11 behind the explicit -d sokol_d3d11 flag.

It also updates the Windows raster/readback path so default Windows OpenGL does not accidentally use D3D11-only readback, and the D3D11 path uses the proper backend when enabled.

Details

• Adds generated built-in shader descriptors from shaders/gui_builtin.glsl.
• Routes built-in pipelines through *_shader_desc(gfx.query_backend()).
• Keeps runtime custom shaders as OpenGL/Metal only for now.
• Documents the D3D11 custom-shader fallback behavior.
• Uses sapp.glue_environment().defaults for render-target formats instead of direct enum casting.
• Adds Windows codegen coverage for default Windows vs -d sokol_d3d11 readback routing.
• Extends the optional Windows smoke workflow with D3D11 compile probes.
• CI now runs the generated-shader freshness check with GUI_CHECK_GENERATED_SHADERS=1 and triggers on shader source changes.
• Removes the old duplicated built-in GLSL/Metal shader strings now replaced by the generated shader header; the remaining GLSL/Metal sources are only for the runtime custom Shader API.

Note: the generated built-in shader header now targets the current Sokol shader set: glsl410 for desktop GLCORE, hlsl4 for D3D11, and metal_macos for macOS Metal. Runtime custom Shader.glsl remains GLSL 3.3, that path is separate from the generated built-in shaders.

Validation

• v -no-parallel test .: 72 passed / 1 skipped / 73 total
• GUI_CHECK_GENERATED_SHADERS=1 v -no-parallel test _shaders_test.v: passed
• v -no-parallel test _sokol_formats_test.v _windows_readback_codegen_test.v: passed
• v -no-parallel test _render_test.v _render_filters_test.v _print_pdf_test.v _print_integration_test.v nativebridge/_readback_abi_test.v: passed
• Windows generated-C probes for default Windows and -d sokol_d3d11: passed
• v check-md docs/SHADERS.md docs/WINDOWS.md shaders/README.md: passed
• git diff --check: passed

Windows Testing Requested

D3D11 is still opt-in in this PR. Before making it the default Windows path, it should be tested on real Windows with -d sokol_d3d11, especially:

• examples/showcase.v
• examples/gradient_demo.v
• examples/shadow_demo.v
• examples/blur_demo.v
• examples/image_clip.v
• examples/svg_demo.v
• examples/printing.v
• examples/custom_shader.v fallback behavior

[chatgpt-codex-connector]

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[GGRei]

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: c1731c18f1

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