Refactor depth buffer rendering

[mvaligursky]

• refactor to better integrate with the engine
• implement support for WEBGL_depth_texture: Implement support for WEBGL_depth_texture extension #2856
• fix: depth buffer is not working correctly after adding layers and removing them again #2932
• fix: ERROR: FRAMEBUFFER_INCOMPLETE_ATTACHMENT when resizing window and depth buffer is rendering. Repro: Post effects engine example with debug engine [this seems fixed in Refactored the grab pass and scene depth rendering #4244]

[Maksims]

Not sure in details how it works now, but recently developer had to do this:

1. Render World Layer into RT.
2. Do few passes to blur texture into another RT.
3. Render Glass Layer (with previously blurred texture) from the respecting actual depth of the World Layer, so it occludes correctly.

And I think it was not smooth or difficult to do such thing efficiently, avoiding extra depth pre-pass on Glass Layer, so to avoid unnecessary drawcalls.

[YessicaSD]

Help!!! I have created SSAO post effect and It works fine in windows but in Mac it triggers the ERROR: FRAMEBUFFER_INCOMPLETE_ATTACHMENT (although it shows the effect)

[mvaligursky]

this is our attempt to have good quality SSAO .. #3092

[YessicaSD]

I've made some changes and it doesn't crush but it seams that the depth buffer is not clearing.
I followed this project's structure -> https://playcanvas.com/project/642131/overview/depthmap-access

[mvaligursky]

if you need support, please post on the forum: http://forum.playcanvas.com/
This is to post issues with the engine code for us to work on.

[mvaligursky]

I tried to use Webgl1 extension WEBGL_depth_texture when refactoring the depth rendering in #4244, to avoid having to re-render meshes from layers before the Depth layer.
This can be done when the camera is rendering to a render target, as using the extension we can then copy the depth using shader into a texture (similarly as done in the Viewer: https://github.com/playcanvas/model-viewer/blob/main/src/read-depth.ts). The problem is when we're rendering straight into a default framebuffer - I don't see a way to copy out the depth from it, as it cannot be accessed as a texture and used in the shader. Specifically, I tried calling this to obtain the depth attachment (texture)

gl.bindFramebuffer(gl.FRAMEBUFFER, null);
const data = gl.getFramebufferAttachmentParameter(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT,
                            gl.FRAMEBUFFER_ATTACHMENT_OBJECT_NAME);

but it results in
WebGL: INVALID_OPERATION: getFramebufferAttachmentParameter: no framebuffer bound

[mvaligursky]

Closing this, as the remaining work was for WebGL1 which is no longer applicable due to WebGL1 support getting removed here: #6666