spine-ts branch: mip maps not supported

[xoorath]

Hey,

if filter: MipMapLinearLinear,Linear is used in an atlas file, an optional flag to enable mip mapping is never passed along to the texture loader. I have a PR I'll send in a second here, I expect the design will not be up to snuff, but I'll link it to this issue so there's at least a working PR that will show the fix.

The practical impact of this issue is that when MipMapLinearLinear,Linear is used, we will get an error from webgl and the textures will appear black. If we switch to Linear,Linear everything looks correct, but the sampling of scaled down sprites appears very jagged.