use Impl for unix vcl plugins

Change-Id: I110df787e1d4b817ab6e682fd9bf50a24332fafd
we need to export the SalGraphicsImpl class

Change-Id: I569a1aacdb7a87b0e8045863a8569520bd0df09a
no need for these methods to be virtual

Change-Id: I23413704b27474f6a59121bb1cbf24ee7484acd2