fix: apply mipmap fixed resolution accordingly to fixed and largest resolutions

[f0nar]

Hi

Sorry that I am coming without an issue but I think PR is pretty simple so we can start discussion here.

I need to generate fixed resolution other then 1. After migration to assetpack 1.2 (after 0.8) I found out that fix tag will not change asset resolution (size) but only naming.
I mean configs like

  resolutions: {
    low: 0.5,
    default: 1,
  },
  fixedResolution: 'low'

with fix tag on asset will copy asset as is without name or size changing.

I can reach needed result with config mentioned before but without fix tag applied but then redundant unchanged asset appear as output for default option.

You can reproduce it with this test by adding fix tag here and adding default: 1 option here
Note: currently this test is overlapped by this one. I dont know if it made on purpose but you need rename one of them to reproduce

Thanks =)

[f0nar]

@Zyie could you check this PR? I cannot find a way to assign it to you