component tiles throws error if camera not defined before 3d tiles entity

[kfarr]

issue: when using with google tiles, if you attempt to place the entity with 3d tiles in an a-frame scene before the camera is defined in index.html you will get an error if you are referencing a camera element that is further down in the html.

psuedocode:
<a-camera id="cam">
<a-tiles entity camera=#cam>
good!

but
<a-tiles entity camera=#cam>
<a-camera id="cam">
bad!

instead: if the camera is not defined, the 3dtiles component should wait until the entire scene is loaded to see if one will be created instead of throwing an error immediately