WebGLRenderer: bugfix when reading pixel in MRT

[andredsm]

Description

This PR fixes a bug when reading pixels from MRT. When using readRenderTargetPixels or readRenderTargetPixelsAsync, the code checks whether the WebGL context can read a pixel from the requested texture:

https://github.com/mrdoob/three.js/blob/1246386f547c5b4e64d3705debfe5a22f18cc91c/src/renderers/WebGLRenderer.js#L2916C1-L2921C7

This is done in textureFormatReadable function by using gl.getParameter( gl.IMPLEMENTATION_COLOR_READ_FORMAT ). Nonetheless, at this point, we did not define the readBuffer yet, so the result will always return the format from COLOR_ATTACHMENT0, raising some errors when I attempt to read pixels from other color attachments. The fix was basically moving the readBuffer calling function before this check.

An example of this bug: https://codepen.io/andredsm/full/jEqXVme. Check the console to see the error. In the code of this example, if you comment the second read, everything works, since we are only reading from the COLOR_ATTACHMENT0.

Finally, I made this pure WebGL2 example showing that the output from gl.getParameter( gl.IMPLEMENTATION_COLOR_READ_FORMAT ) is affected by the state of gl.readBuffer: https://codepen.io/andredsm/pen/pvyqEqV.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.9 83.23	350.9 83.23	-2 B -3 B
WebGPU	616.22 171.03	616.22 171.03	+0 B +0 B
WebGPU Nodes	614.82 170.77	614.82 170.77	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	483.14 118.07	483.14 118.07	-2 B -4 B
WebGPU	687.06 186.63	687.06 186.63	+0 B +0 B
WebGPU Nodes	636.9 173.84	636.9 173.84	+0 B +0 B

[Mugen87]

@repalash Does this change look good to you?