Use a generic plot mission image instead of default grey XCOM logo

[furudee]

Dropship mission briefing screen displays an image of the objective parcel/PCP which are defined in XComMaps.ini in arrMapImageDefs array
For example, a mission with a vanilla geneclinic objective parcel will fetch an image path from the array item
arrMapImageDefs=(MapName="lg_GeneClinic_01" , ImagePath="UILibrary_MissionImages.Mission_lg_GeneClinic_01", Biome="")

Modded maps may not append an entry for their custom objective parcels, so the game will display a default image assuming it's at index zero of the array

;DEFAULT IMAGE loads if no image is found below
arrMapImageDefs=(MapName=""	, ImagePath="UILibrary_Common.Xcom_default", Biome="")

The array also has generic images for plot types which are used for TLE skirmish menu and multiplayer

So my suggestion is that instead of showing the lame grey default image, it will show a generic plot type image when it fails to find a proper image for an objective