The bottom interface layers were not right when "independent support layer height" is enabled.
This commit ensures there are always 2 bottom interface layers and the gap
is not less than specified. However, the gap may be slightly larger.
Jira: STUDIO-3842, STUDIO-2138
Github: #2127
Change-Id: Ifd8fbc4c7bc6dd92f2534fdd0179458a9e93c79a
(cherry picked from commit edcdad162ef68ab94f8d1e66c225c2617f43e00e)