Fix skinning precision issue on iOS

On iOS, by default the bones that are sampled from the bone texture look
like they are using mediump. This can result in noticeable jitter or
other more severe issues depending on the bone transformation matrix.

Fix this by explicitly declaring the sampler as highp.

src/renderers/shaders/ShaderChunk/skinning_pars_vertex.glsl.js

@@ -6,7 +6,7 @@ export default /* glsl */`
 
 	#ifdef BONE_TEXTURE
 
-		uniform sampler2D boneTexture;
+		uniform highp sampler2D boneTexture;
 		uniform int boneTextureSize;
 
 		mat4 getBoneMatrix( const in float i ) {