…in ps_yuv_image.

Use WR_FEATURE_TEXTURE_EXTERNAL for samplerExternalOES support.
Use WR_FEATURE_TEXTURE_RECT for sampler2dRect support.
Use WR_FEATURE_NV12 for NV12 format support.
The WR_FEATURE_TEXTURE_EXTERNAL and WR_FEATURE_TEXTURE_RECT are exclusive.