PMREMGenerator: Fix viewport/scissor settings.

[Mugen87]

This should fix the broken PMREMGenerator on dev (for computer with device pixel ration > 1).

@elalish When using WebGLRenderer.setViewport(), the viewport is automatically multiplied with the device pixel ratio e.g.:

three.js/src/renderers/WebGLRenderer.js

Line 467 in 9518d5e

state.viewport( _currentViewport.copy( _viewport ).multiplyScalar( _pixelRatio ).floor() );

However, when you directly set WebGLRenderTarget.viewport and WebGLRenderTarget.scissor, it's not necessary to divide through the device pixel ratio because it's not respected in WebGLRenderer.setRenderTarget().

three.js/src/renderers/WebGLRenderer.js

Lines 2664 to 2665 in 9518d5e

	_currentViewport.copy( renderTarget.viewport );
	_currentScissor.copy( renderTarget.scissor );

[WestLangley]

Pixel ratio does not apply to WebGLRenderTarget.

[Mugen87]

Updated the PR to resolve the dependency to Vector4.

[Mugen87]

Merging and updating builds for further testing.

[Mugen87]

The issue is fixed on my Pixel, too.

[mrdoob]

Thanks!

[elalish]

@Mugen87 Thanks! Sorry the DPR thing caught me yet again...