trunk: embedded bitmap images broken ("Linked image not found") (rev >= 12942)

[su_v]

Steps to reproduce:
1) launch current trunk (default prefs, default new doc)
2) Import a bitmap image (choose 'Embed' on import')

Expected result:
Bitmap image is embedded into current document, and rendered on canvas.

Actual result:
Bitmap image is embedded into current document, but the fallback image 'linked image not found' is rendered on canvas.

Additional notes:
- Also affects rendering of existing documents with embedded bitmap images.
- The formatting of the embedding has changed with rev >= 12939 (which AFAICT affects how the embedded bitmap image is presented in the XML Editor).

Based on tests with archived builds on OS X 10.7.5:
- not reproduced with rev <= 12940
- reproduced with rev >= 12943
this regression seems to have been introduced in
- Revision 12942: Clean up sp-image's read_image, remove duplicate code
  <http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/12942>

[su_v (suv-lp)]

David was faster - linking as duplicate to bug #1270335.