Add support for 3K and 6K to the Dome Render Texture

[SR2610]

Altered the dropdown inside of DomeRendererEditor.cs to allow for 3072x3072 and 6144x6144 render texture sizes.

[hybridherbst]

Thanks! Did you happen to measure the performance difference between 3k and 4k, and between 6k and 8k?
(On many types of GPUs the backing texture will likely be the next power of two size, so I'd be curious how it went in your tests)

[SR2610]

I've only tested it on one machine with one simple project but 3K performs better than 4K but worse than 2K and the 6K is similar where it is between the 4K and 8K in performance.
This is on an RTX 2080 so I would also be curious to see if this is how it behaves on other GPUS too.