Fix subsampling rounding in rdopt_utils.h

[y-guyon]

Fixes https://github.com/webmproject/CrabbyAvif/blob/c3136f98215bcb49e11581fa32f1703fad9d9740/tests/av2.rs#L35

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I found the pattern used in this change in a few other places:

avm/av2/common/alloccommon.c

Lines 349 to 350 in cd2ba54

	const int plane_w =
	((frame_w + ss_x) >> ss_x) + 2 * RESTORATION_BORDER_HORZ;

avm/avm_scale/generic/yv12config.c

Lines 145 to 146 in cd2ba54

	ybf->uv_crop_width = (width + ss_x) >> ss_x;
	ybf->uv_crop_height = (height + ss_y) >> ss_y;