Avoid tesseract writing Pix out/reading them back.

[robinwatts]

By default, when we ImageData::SetPix, we write the data out as a
PNG, just to read it back in to get a compressed buffer of data.
We then use this to generate a new Pix.

In builds of Tesseract on systems where we don't have temp files,
writing files out is problematic.

Not only that, but compressing/uncompressing is slow, and on minimal
builds of leptonica, where we've disabled the format writers to reduce
memory footprint, we get no compression anyway.

In such cases, it'd be far nicer just to keep the original Pix as
the internal data.

Also, when recovering the pixmap from the ImageData, if we know we're
only going to read from the data, we can avoid duplicating it and
just use the original. This is exactly the case when GRAPHICS_DISABLED
is set.

So, introduce a TESSERACT_IMAGEDATA_AS_PIX predefine that we can use
to cause the internal data to be a Pix rather than a compressed
buffer.

Given we don't do compression, and they were writing to memory,
this was all just more effort than we needed.

Also, if we're using GRAPHICS_DISABLED, we might as well just
pixCopy rather than pixClone as only the scaler uses this.

[robinwatts]

I am looking into using Tesseract with Ghostscript. These are some changes that I made locally to smooth operation. For systems without fmemopen (such as windows), tesseract drives leptonica to save images out, just so it can then read the compressed version back in again.

This commit enables a build define that avoids that.

[zdenop]

I like the concept to build minimal tesseract (I tried to do it some time ago AFAIK only zlib was needed).