The m_imageData buffer in the BITMAP_BASE is an
internal implementation - the public API to persist the
data to a stream appears to be SaveImageData - so use that,
which hides the implementation (and generates the image data
"live" when needed).
Remove the public access to the m_imageData buffer as it isn't
needed for public use, and also is misleading.
Also break out the formatting of the data into KICAD_FORMAT,
as it's currently replicated in eeschema/pcb/pagelayout
formatting code.
Fixes: https://gitlab.com/kicad/code/kicad/-/issues/19772
John Beard
636c6c4efb