[BUG] ImageBuf not working with in-memory DDS files

[MiroslavR]

I am trying to create ImageBuf using an IOMemReader pointing to an in-memory DDS file. However, ImageBuf::init_spec fails with Invalid DDS header, possibly corrupt file. ImageInput::open works fine with IOMemReader. The problem seems to be specific to DDS format, I could not reproduce with PNG.

Code for reproduction:

#include <OpenImageIO/filesystem.h>
#include <OpenImageIO/imagebuf.h>

#include <fstream>
#include <iostream>

namespace OIIO = OpenImageIO_v2_4;

void fails(const std::string& ddsData)
{
	OIIO::Filesystem::IOMemReader memReader(ddsData.data(), ddsData.size());
	OIIO::ImageBuf buf("not_existing_on_disk.dds", 0, 0, nullptr, nullptr, &memReader);

	if (!buf.init_spec(buf.name(), 0, 0))
	{
		std::cout << "init_spec failed: " << buf.geterror() << std::endl;
		return;
	}

	std::cout << "init_spec succeeded; size: " << buf.spec().width << " x " << buf.spec().height << std::endl;
}

void works(const std::string& ddsData)
{
	OIIO::Filesystem::IOMemReader imageReader(ddsData.data(), ddsData.size());
	auto in = OIIO::ImageInput::open("not_existing_on_disk.dds", nullptr, &imageReader);
	if (!in)
	{
		std::cout << "ImageInput::open failed" << std::endl;
		return;
	}

	std::cout << "ImageInput::open succeeded; size: " << in->spec().width << " x " << in->spec().height << std::endl;
}

int main()
{
	std::fstream fs("example.dds");
	const std::string ddsData((std::istreambuf_iterator<char>(fs)), std::istreambuf_iterator<char>());

	fails(ddsData);
	works(ddsData);

	return 0;
}

The result I get:

init_spec failed: Invalid image file "not_existing_on_disk.dds": Invalid DDS header, possibly corrupt file
ImageInput::open succeeded; size: 512 x 256

The result I expect:

init_spec succeeded; size: 512 x 256
ImageInput::open succeeded; size: 512 x 256

I am on Debian Testing, using OIIO version 2.4.14 (distro package). Here's the whole project along with example.dds:
openimageio_test.zip

[jessey-git]

I'll take a look at this one. I think it's pretty simple so I'll send a PR around tomorrow for it.