compiler: support center in d2-config

[x-delfino]

This PR fixes support for center in d2-config variables. Looks like it was only partially implemented but should be supported per documentation.

Test file:

vars: {
  d2-config: {
    center: true
  }
}

x -> y

before change:

preserveAspectRatio="xMinYMin meet"

after change:

preserveAspectRatio="xMidYMid meet"

[alixander]

good catch, ty!