Game pauses for ~4 seconds when selecting mode

[Poobslag]

After the new customer graphics in #62, game load times increased significantly from about 1-2 seconds to about 4-5 seconds, and that's on my computer. On other computers this might be even slower.

This is most likely to adding all of the new textures. This problem will only get worse over time if we add more and more graphics for the different bugs. We should pursue different options for speeding this up:

• Cache textures in a singleton so that they only load once on startup -- not every time a game starts
• Or the other direction, load textures in the background instead of loading them all up-front
• Make the textures faster to load; maybe instead of 1024x1024 images, we can scale them down to 256x256.
• Maybe using a tool like "PNG crush" to shrink the PNGs could help. It's doubtful, but there's a chance that if they become indexed PNGs it'll clue Godot in that it can be more efficient with them.
• Maybe GLES2 loads textures faster?
• This is a pretty simple problem so others may have suggestions online. These people think disabling etc2comp could improve performance. These people wrote some sort of custom 'texture caching' thing. These people think having too many unique shaders takes time to compile.
• Maybe tweaking some of our texture filtering options could improve performance too, such as mipmaps or 'lossy quality'. I'm not certain whether we rely on mipmaps.

[Poobslag]

Fixed in #65 , resources are now loaded in the background.