This improves my PC's performance on RS2 Hoth by... 0.1% or so, which I
think is within the margin of error. But this change also cuts down on
boilerplate.

The TextureInfo constructor creates a vector of MipLevels. This could be
good for performance if MipLevels are accessed very often for each
TextureInfo, but that's not the case. Dolphin creates thousands of
TextureInfos per second that it never accesses the mipmap levels of
because there's a hit in the texture cache, and in the uncommon case of
a texture cache miss, the mipmap levels only get looped through once.

To make the common case of texture cache hits as fast as possible, let's
not create a vector in the TextureInfo constructor. This commit
implements a custom iterator for MipLevels instead.

In my testing on the Death Star level of Rogue Squadron 2, this speeds
up TextureInfo::FromStage by 200%, giving an overall emulation speedup
of a bit over 1%. Results on the Hoth level are even better, with
TextureInfo::FromStage being close to 300% faster and overall emulation
being over 4% faster. (Single core, no GPU texture decoding.)

Now only VertexLoader remains... But that one might be tricky.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.