Bitmap class cannot be used with already loaded image data

[stohrendorf]

Consider an atlas image is created on the fly in memory in SRGBA8 format outside of etcpak. The Bitmap class can only be successfully constructed from a file, not from already present image data. When not providing a filename, m_sema is not initialized properly, so that later calls to NextBlock keep blocking, as the Bitmap class is effectively left uninitialized, and thus is not ready for data retrieval. It would be nice if either:

1. the Bitmap class would get an additional constructor to construct it from already present image data, or
2. the m_sema member would be initialized so that calls to NextBlock do not block for processing.

Thank you.

[wolfpld]

Consider an atlas image is created on the fly in memory in SRGBA8 format outside of etcpak.

This is outside the scope of what etcpak aims to be.