Clean up frustum_cull_test example

[rparrett]

• Show instructions and current map type on screen with good contrast in the UI layer

That was my original goal, but I felt like this thing really needed some attention. I am often scratching my head at what it's supposed to be doing when returning to this repo to test.

• Separate code responsible for determining next map type and changing map properties for clarity
• Get helpers::camera::movement out of the First schedule
• Remove "mouse picking" comments not related to this example
• Combine texture handle resources
• Tidy up imports
• Other misc cleanup

A lot of these improvements should be implemented in other examples as well. I think we should consider pruning the hexagon examples first. See #588.