Trace bitmap crashes for all images in master

Steps to reproduce:

• open Inkscape
• open png or jpeg image
• select image
• Path > Trace bitmap

What happened?

Crash

src/display/cairo-utils.cpp:460

ssertion `_pixel_format == PF_GDK' failed.

What should have happened?

This is linked to @pbs3141 patch 797be5a7 which made the sp-image pixbuf imutable. Unfortunatly this means that if an image isn't in the correct format on creation, then the pixbuf can't be changed afterwards to be compatible.

A couple of solutions might be possible:

• Trace bitmap might make a copy (might be bad for memory)
• The Bitmap could be converted at creation time
• The pixbuf could be allowed some flexibility to convert this just in time

Inkscape Version and Operating System:

• Inkscape Version: ...
• Operating System: ...
• Operating System version: ...