Fix failing sun tests with WebGL validation on.

[bagnell]

For #2771.

The framebuffer was incomplete because we render the sun to a texture that is a fraction of the drawing buffer size. For the tests, the size is between 0 and 1. In practice, this will never happen, but I capped the minimum size to 1 for the tests. Since this never failed before, maybe there was an update where the texture width and height must be greater than or equal to 1?

[mramato]

I can confirm the tests are fixed. @lilleyse or @pjcozzi can you merge if you're okay with this change. Thanks.

[pjcozzi]

@bagnell can you add a comment in master about why the min call is there. Also, is there anywhere else that we need this? Sounds like something the post-processing framework will need to support in general.