WebGPURenderer: Fix copyFramebufferToTexture wrong framebuffer binding in WebGLBackend

[RenaudRohlinger]

In the WebGLBackend the copyFramebufferToTexture was not binding the correct framebuffer.

Also, the null binding of the READ_FRAMEBUFFER was throwing a WebGL warning, I removed it as anyway WebGL DRAW_FRAMEBUFFER operation will fallback to read the window-system-provided framebuffer and work just fine. This does another error warning throw on firefox and chrome. /cc @aardgoose

This fix the new webgpu_backdrop_water example (error caused by copyFramebufferToTexture called in ViewportTextureNode).

Before:

After:

This contribution is funded by Utsubo