arrange_windows_r: Round pixels to match reality, fixes calculations.

If the width or height of a container can't be evenly distributed to its
children, then the layout algorithm still thought it got it right (due
to using decimals) which caused a gap of one or more pixels for some
window arrangements.

This is fixed by this patch by first rounding off the width and height
(so that decimals are never introduced) and then adjusting the last
view in a container to fill the remaining pixels (which now is counted
correctly due to the decimals being removed).

Also, due to the way gaps are implemented, an odd sized gap can never be
aligned properly, so just adjust to closest even number.

sway/CMakeLists.txt

@@ -32,6 +32,7 @@ target_link_libraries(sway
 	${PCRE_LIBRARIES}
 	${JSONC_LIBRARIES}
 	${WAYLAND_SERVER_LIBRARIES}
+	m
 )
 
 install(