In OpenGL this is called the (base) internal format which is an
expectation expressed by the client for the minimum supported texel
storage format in the GPU for textures.
Since we store everything as RGBA in a `FloatVector4`, the only thing
we do in this patch is remember the expected internal format, and when
we write new texels we fixate the value for the alpha channel to 1 for
two formats that require it.
`PixelConverter` has learned how to transform pixels during transfer to
support this.
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