Implement WEBGL_depth_texture extension

[jdm]

https://developer.mozilla.org/en-US/docs/Web/API/WEBGL_depth_texture

[jdm]

Right now https://threejs.org/examples/webgl_depth_texture.html doesn't run in Servo. To implement the missing extension, we need to:

• add the new WebIDL from https://www.khronos.org/registry/webgl/extensions/WEBGL_depth_texture/ to components/script/dom/webidls/WEBGL_depth_texture.webidl
• add a new extension implementation in components/script/dom/webgl_extensions/ext/, and make it be supported if the ANGLE_depth_texture GL extension is supported
• add the new formats and data types to the TexFormat/TexDataType enums in components/canvas_traits/webgl.rs
• add the new type values to the list DEFAULT_DISABLED_TEX_TYPES_WEBGL1 in components/script/dom/webgl_extensions/extensions.rs, and enable these types as part of enabling the new webgl extension (WebGLExtensions::enable_tex_type)
• update the implementation of WebGLRenderingContext::validate_tex_image_2d_data to support the Uint32 type
• update the implementation of WebGLRenderingContext::FramebufferTexture2D to support the new allowed target values when this extension is enabled
• update WebGLFramebuffer::texture2d to pass the actual desired target value instead of constants::FRAMEBUFFER, and validate the internal format of the texture is appropriate for non-FRAMEBUFFER targets
• implement the error conditions described in "Errors" in https://www.khronos.org/registry/webgl/extensions/WEBGL_depth_texture/ for the APIs listed

If everything goes correctly, there should be more passing tests in ./mach test-wpt tests/wpt/webgl/tests/conformance/extensions/webgl-depth-texture.html and https://threejs.org/examples/webgl_depth_texture.html should match Firefox's output.

[ivanshen]

I've created the new extension implementation following the webglcolorbufferfloat.rs file. For the new data types, if the idl contained const GLenum UNSIGNED_INT_24_8_WEBGL = 0x84FA;, is this what I should add to the enum TexDataType? Something like UnsignedInt248 = gl::UNSIGNED_INT_24_8_WEBGL?

[jdm]

You will need to use UnsignedInt24_8 = gl::UNSIGNED_INT_24_8.

[jdm]

This extension is used in lots of cables.gl content like https://holon.drastic.net/view/moron/ and https://cables.gl/p/9CHVn1.

[jdm]

We will need to expose this extension's behaviour by default in webgl2 as well.

[jdm]

Playing around with trying to implement this at https://github.com/jdm/servo/tree/depth-texture. texImage2D and framebufferTexture2D no longer trigger WebGL errors, but the actual pixel tests are still failing.