This patch improves the robustness of menu_view when encounting missing
textures, which can happen during development when using styled buttons
and frames. With the patch, menu_view outputs diagnostic messages,
pinpointing the problem.
The patch also updates the texture handling to use the 'File_content'
utility and the VFS for obtaining PNG images.
Issue #3629
This patch fixes a problem that unsurfaced by the commit "menu_view: API
transition", which changed the class layout of the 'Png_image' so that
the 'Read_struct' pointer is no longer equal to the 'Png_image' pointer.
