ImfFrameBuffer.h not compatible with C++98 code

[clopez]

Since commit 119eb2d ImfFrameBuffer.h includes cstdint

And cstdint is a C++11 header, so now including ImfFrameBuffer.h on any C++98 code will fail to build.

A possible fix is including stdint.h instead of cstdint on ImfFrameBuffer.h

See for example https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/issues/1089 for a build failure this has caused

[cary-ilm]

The Technical Steering Committee has made the decision to adopt C++11 going forward and drop support for C++98 in the interest of clarity in the code and ease of maintenance. This means that 2.4.0+ releases won't compile on older systems, at least not without local changes.