Add option to preserve alpha channel in export (closes #661)

Default off cause it being on by default had confused quite a few people.

src/main/java/com/replaymod/render/gui/GuiExportFailed.java

@@ -90,6 +90,7 @@ public class GuiExportFailed extends GuiScreen {
                     oldSettings.getBitRate(),
                     oldSettings.getOutputFile(),
                     oldSettings.isRenderNameTags(),
+                    oldSettings.isIncludeAlphaChannel(),
                     oldSettings.isStabilizeYaw(),
                     oldSettings.isStabilizePitch(),
                     oldSettings.isStabilizeRoll(),