WebGLRenderer: Reset mask before clearing stencil.

[OndrejSpanel]

Description

When the last object rendered before the clear uses stencilWriteMask, which may happen when doing multiple passes with such materials, or when using some render targets, the clear will not clear bits disabled by the mask.

See https://developer.mozilla.org/en-US/docs/Web/API/WebGLRenderingContext/clear

The scissor box, dithering, and buffer writemasks can affect the clear() method

This PR resets mask before doing the clear

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
647.6 kB (160.8 kB)	647.6 kB (160.8 kB)	+47 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

Filesize dev	Filesize PR	Diff
440.9 kB (106.9 kB)	440.9 kB (106.9 kB)	+47 B

[Mugen87]

Can you please provide a fiddle that demonstrates the error the PR fixes?

[OndrejSpanel]

I can try, but it is not trivial. In my case the error was happening randomly in a large scene where stencil was used for object outlining. Depending on object order the error appeared sometimes (old outlines sticking on the screen).

[OndrejSpanel]

Here is the issue repro: https://jsfiddle.net/OndrejSpanel/531fa2e8/116/

Steps:

1. Render cyan sphere, writing 4 to stencil
   

2. Render green sphere with a stencil test value == 4, setting the write mask to zero (any other render call would do, just set the write mask so that bit 4 is not cleared later):
   

3. Clear the backbuffer---- here the issue happens. The stencil is not cleared.

4. Render the green sphere again with a stencil test value == 4 on a freshly cleared backbuffer:
   
   Nothing should appear, as the stencil buffer should be zero after clear. It is however not cleared and the same shape as in step 2 appears.