Visual Mode: changing brightness now affects sector brightness, holding Alt will change local brightness

Visual Mode: fixed a problem where sloped 3D floors with an alpha value of less than 128 were not shown at all instead of fully opaque. Fixes #954

Draw Curve Mode, Draw Ellipse Mode, Draw Rectangle Mode: added option to place things at the designated vertex positions instead of drawing geometry (#956)

Point Thing at Cursor: check for a thing's fixedrotation rather than its category's arrow setting (#957)

Visual Mode: when applying camera rotation and pitch to things a status message will now be displayed. Fixes #955

Visual Mode: fixed an issue where trying to apply a rotation to things that were not seen yet would cause a crash

Fixed a problem where not selected geometry and things could not be dragged when the "don't move selection if any part of it is outside of map boundary" option was enabled

Visual Mode: fixed a problem where the selection order was not kept when synchronized selection between visual and classic modes was enabled. Fixes #938

Visual Mode: fixed a problem where things directly touching the top of 3D floors were rendered with the incorrect brightness. Fixes #940

Visual Mode: changes to Eternity Engine's upper/lower texture skewing definitions are now shown in Visual Mode

Visual Mode: fixed a crash when toggling visual vertex slope picking and the map contains double-sided linedefs without a front sidedef. Fixes #928

- fixed a rare condition where skewed mid textures were not clipped correctly
- fixed a problem where translucent mid textures were not skewed correctly
- fixed a problem where sprites were skewed unintentionally