vp9/10: improve support for render_width/height.

In the decoder, map this to the output variable vpx_image_t.r_w/h.
This is intended as an improved version of VP9D_GET_DISPLAY_SIZE,
which doesn't work with parallel frame decoding. In the encoder,
map this to a codec control func (VP9E_SET_RENDER_SIZE) that takes
a w/h pair argument in a int[2] (identical to VP9D_GET_DISPLAY_SIZE).

Also add render_size to the encoder_param_get_to_decoder unit test.

See issue 1030.

Change-Id: I12124c13602d832bf4c44090db08c1009c94c7e8