cogl-bitmap-pixbuf: Avoid copying the buffer in more circumstances
cogl-bitmap-pixbuf: Avoid copying the buffer in more circumstances

When loading an RGB image GdkPixbuf will pad the rowstride so that the
beginning of each row is aligned to 4 bytes. This was causing us to
fallback to the code that copies the buffer. It is probably safe to
avoid copying the buffer if we can detect that the rowstride is simply
an alignment of the packed rowstride.

This also changes the copying fallback code so that it uses the
aligned rowstride. However it is now extremely unlikely that the
fallback code would ever be used.

gcampax/cogl,Distrotech/cogl,djdeath/cogl,Distrotech/cogl,gcampax/cogl,collects/cogl,gcampax/cogl,djdeath/cogl,spatulasnout/cogl,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl-android,collects/cogl,djdeath/cogl,spatulasnout/cogl,Distrotech/cogl,djdeath/cogl-android,gcampax/cogl