Override stencilAttachmentPixelFormat to maintain appropriate state

We dont end the encoder in EncoderManager when we move from stencil based opcodes to other opcodes because doing so might decrease the performance. Also for stencil based opcodes we set stencilAttachmentPixelFormat in updatePipelineState.

But this might cause issues when we enable Metal API validation and hit non stencil based opcodes after stencil operation in same encoder. Override the stencilAttachmentPixelFormat to maintain appropriate state until we end the encoder.