render/pixman: Remove half-pixel shift

Reverts 77006e5565.  It turns out this
change really breaks the labwc drop-shadow implementation (which relies
on scaling up single-pixel-wide buffers).

After reverting this there's still something subtley wrong with scaling
pixel locations (shown by `weston-scaler -b`) but I can't see an obvious
way to make both weston-scaler and labwc drop-shadows work in the same
way with pixman and gles2. I'll have a harder look at this but in the
mean time reverting this patch seems to make things less broken on
average.

render/pixman/pass.c

@@ -155,13 +155,6 @@ static void render_pass_add_texture(struct wlr_render_pass *wlr_pass,
 		pixman_transform_translate(&transform, NULL,
 			pixman_int_to_fixed(src_box.x), pixman_int_to_fixed(src_box.y));
 
-		// GPUs have a half pixel shift compared to pixman because GPU coordinates refer
-		// to the centre of each pixel rather than its top-left.  This makes no difference
-		// when there's no transforms as we still copy the correct pixel across.  But when
-		// we do scaling it does make a visible difference, so account for it here.
-		pixman_transform_translate(&transform, NULL,
-			-pixman_double_to_fixed(0.5), -pixman_double_to_fixed(0.5));
-
 		pixman_image_set_transform(texture->image, &transform);
 
 		switch (options->filter_mode) {