Add ability to purge textures from RAM if they are read-only
Make use of both for constellation images to reduce the memory footprint
(cherry picked from commit 7f0c92430f)
* Cleanup
* CMake cleanups
* Update current year
* Update copyright header
* Use script to return list of all modules
* Update credits, license and ghoul
* Make height map fallback layer work again
* Add documentation to joystick button bindings
* Removed grouped property headers
* Add new version number constant generated by CMake
* Make Joystick deadzone work properly
* Change the startup date on Earth to today
* Fix key modifier handling
* Add debugging indices for TreeNodeDebugging
* Fix script schedule for OsirisRex
* Do not open Mission schedule automatically
* Upload default projection texture automatically
* General code cleanup
* Fix check_style_guide warnings
* Remove .clang-format
* MacOS compile fixes
* Clang analyzer fixes
* Adding ScreenSpaceSpout object to display spout textures in screen space
* Enable the clearing of option properties
* Fix crash in ScreenSpaceImageLocal specification
* Split RenderablePlane into RenderablePlaneImageLocal and RenderablePlaneImageOnline
* Add RenderablePlane Spout
* Rename Texture parameter in ScreenSpaceImageOnline into URL
Closes#518
