PMREMGenerator: Fix replacing render targets with different heights

[JordyvanDortmont]

Related PR: #23322

Description

When calling fromEquirectangular (or _fromTexture in general) without a render target (default null) for a second time with a texture with a different height than the texture for the first call, but the same width, the _pingPongRenderTarget is not replaced. After which the lighting from setting the Scene.environment to the render target texture fails, in two different ways, depending on the order (larger or smaller height first/second).

[WestLangley]

/ping @elalish ... just checking... is this correct?

const width = 3 * Math.max( this._cubeSize, 16 * 7 );
const height = 4 * this._cubeSize;

[elalish]

@WestLangley indeed it is; it's much like the internal storage of a mipmapped cubemap, but with the addition of 7 16x16 cubemaps in a horizontal stripe at the top of the image for the higher roughness values. The max ensures there's enough room for them if the input cube is small.

This change looks good to me.