Check for TrueColor support in x0vncserver

Colour map support was removed in b6b4dc6, but x0vncserver didn't
properly check if the X server required it.
diff --git a/unix/x0vncserver/XPixelBuffer.cxx b/unix/x0vncserver/XPixelBuffer.cxx
index f464182..4769b65 100644
--- a/unix/x0vncserver/XPixelBuffer.cxx
+++ b/unix/x0vncserver/XPixelBuffer.cxx
@@ -41,7 +41,7 @@
   format = PixelFormat(m_image->xim->bits_per_pixel,
                        m_image->xim->depth,
                        (m_image->xim->byte_order == MSBFirst),
-                       m_image->isTrueColor(),
+                       true,
                        m_image->xim->red_mask   >> (ffs(m_image->xim->red_mask) - 1),
                        m_image->xim->green_mask >> (ffs(m_image->xim->green_mask) - 1),
                        m_image->xim->blue_mask  >> (ffs(m_image->xim->blue_mask) - 1),