template is bold
sample data is italicized

Issue Description:
With any dynmap cave-view shader, Dynmap will not properly render caves if barrier blocks are in the way. Its hard to explain, but if you look at the screenshot below, you will see what I mean. I have a world map that is completely surrounded by barriers on the sides and the top. It looks like those barriers are interfering with the cave view generation.

This is unexpected because barriers are fully transparent blocks and I think support for ignoring them should be added.

• Dynmap Version: 3.4-beta

• Server Version: Paper spigot 1.12.2

• Pastebin of Configuration.txt: https://pastebin.com/5SETL3z2

• Other Relevant Data/Screenshots:
  

• Steps to Replicate:

1. Make a box out of barriers.
2. Use default settings to render a cave view perspective

[X] I have looked at all other issues and this is not a duplicate
[X] I have been able to replicate this

Side note: In my particular case, since the barriers were at build height I was able to setup a custom perspective to bypass this problem by defining a "maximumheight" option.