WebGLRenderer: Fixes Mesh rendering drawArrays call when position attribute array itemSize is not 3.

Changes suggested for #5964 resolves #5042 and supersedes #5952, #5953
and #5964

WebGLRenderer: Made renderObjects(), renderObjectsImmediate() and setMaterial() private with '_' prefix.

WebGLRenderer: Parameter overrideMaterial in renderObjects() and renderObjectsImmediate() should not be optional.

WebGLRenderer: Fixed bug where blending was always used with overrideMaterial -- even when material.transparent = false. This did not match material behavior when overrideMaterial was not used. Also refactored unnecessary useBlending parameter, as blending is only used if material.transparent === true.

It copies RGBA pixels from WebGLRenderTarget to unsigned byte array. Array must be of a size 4 * width * height.
This is useful when doing offscreen rendering and you need to access pixels directly.
Also it is useful when doing windowless rendering (i.e. using node.js) - then this is the only way to get rendered image.

Fixed non-indexed line arrays not using offset start index. Also removed needless variable assignment from THREE.Line case and THREE.PointCloud case.

Fixed offset particle drawing always starting at 0 index. Looks like it was copy'n'paste typo.

When multiple morphTargets where in the same scene, this optimization resulted in a mix of morphTargets from different animations with different sizes.