Src: Set WebGLRenderTarget texture.flipY to false

[WestLangley]

PMREM CubeUV textures have .flipY set to true. But .flipY does not apply to render target textures.

I think the most appropriate fix is for WebGLRenderTarget to set .flipY to false.

/ping @Mugen87

[WestLangley]

Continuing to set .flipY to false in other derived classes is OK, but one could argue it would be less confusing to set .flipY only when necessary. I do not have a strong opinion either way.

[Mugen87]

So this change is only for clarity, right? Although flipY is not apply to render target textures, it less confusing to set its default to false.

[WestLangley]

Yes. I exported a PMREM texture and imported it, and the .flipY flag changed from true to false. I then discovered it was the WebGLRenderTarget class that failed to set the flag.

[mrdoob]

Thanks!