WebGLRenderer: Fixed missing DepthStencilFormat import.

src/renderers/WebGLRenderer.js

-import { MaxEquation, MinEquation, RGB_ETC1_Format, RGBA_PVRTC_2BPPV1_Format, RGBA_PVRTC_4BPPV1_Format, RGB_PVRTC_2BPPV1_Format, RGB_PVRTC_4BPPV1_Format, RGBA_S3TC_DXT5_Format, RGBA_S3TC_DXT3_Format, RGBA_S3TC_DXT1_Format, RGB_S3TC_DXT1_Format, SrcAlphaSaturateFactor, OneMinusDstColorFactor, DstColorFactor, OneMinusDstAlphaFactor, DstAlphaFactor, OneMinusSrcAlphaFactor, SrcAlphaFactor, OneMinusSrcColorFactor, SrcColorFactor, OneFactor, ZeroFactor, ReverseSubtractEquation, SubtractEquation, AddEquation, DepthFormat, LuminanceAlphaFormat, LuminanceFormat, RGBAFormat, RGBFormat, AlphaFormat, HalfFloatType, FloatType, UnsignedIntType, IntType, UnsignedShortType, ShortType, ByteType, UnsignedShort565Type, UnsignedShort5551Type, UnsignedShort4444Type, UnsignedByteType, LinearMipMapLinearFilter, LinearMipMapNearestFilter, LinearFilter, NearestMipMapLinearFilter, NearestMipMapNearestFilter, NearestFilter, MirroredRepeatWrapping, ClampToEdgeWrapping, RepeatWrapping, FrontFaceDirectionCW, NoBlending, BackSide, DoubleSide, TriangleFanDrawMode, TriangleStripDrawMode, TrianglesDrawMode, NoColors, FlatShading, LinearToneMapping } from '../constants';
+import { MaxEquation, MinEquation, RGB_ETC1_Format, RGBA_PVRTC_2BPPV1_Format, RGBA_PVRTC_4BPPV1_Format, RGB_PVRTC_2BPPV1_Format, RGB_PVRTC_4BPPV1_Format, RGBA_S3TC_DXT5_Format, RGBA_S3TC_DXT3_Format, RGBA_S3TC_DXT1_Format, RGB_S3TC_DXT1_Format, SrcAlphaSaturateFactor, OneMinusDstColorFactor, DstColorFactor, OneMinusDstAlphaFactor, DstAlphaFactor, OneMinusSrcAlphaFactor, SrcAlphaFactor, OneMinusSrcColorFactor, SrcColorFactor, OneFactor, ZeroFactor, ReverseSubtractEquation, SubtractEquation, AddEquation, DepthFormat, DepthStencilFormat, LuminanceAlphaFormat, LuminanceFormat, RGBAFormat, RGBFormat, AlphaFormat, HalfFloatType, FloatType, UnsignedIntType, IntType, UnsignedShortType, ShortType, ByteType, UnsignedShort565Type, UnsignedShort5551Type, UnsignedShort4444Type, UnsignedByteType, LinearMipMapLinearFilter, LinearMipMapNearestFilter, LinearFilter, NearestMipMapLinearFilter, NearestMipMapNearestFilter, NearestFilter, MirroredRepeatWrapping, ClampToEdgeWrapping, RepeatWrapping, FrontFaceDirectionCW, NoBlending, BackSide, DoubleSide, TriangleFanDrawMode, TriangleStripDrawMode, TrianglesDrawMode, NoColors, FlatShading, LinearToneMapping } from '../constants';
 import { Matrix4 } from '../math/Matrix4';
 import { WebGLUniforms } from './webgl/WebGLUniforms';
 import { UniformsUtils } from './shaders/UniformsUtils';