trunk: mesh gradient tool cannot assign colors to stops, crash when dragging a stop (rev >= r12532)

[su_v]

Steps to reproduce:
0) build trunk with mesh tool enabled
1) launch trunk (default prefs, default new doc)
2) draw a reactangle
3) switch to the mesh gradient tool
4) defined a 3x3 mesh (controls bar)
5) double-click the rectangle with the mesh tool

6) try to assign different colors to the inner stops
--> only the first one updates the color, later ones don't
7) grab the node last selected and drag it
--> crash:

SPGradient::rebuildArray(): reffed array NOT IMPLEMENTED!!!

** (inkscape:76406): WARNING **: SPGradient::rebuildArray() called for non-mesh gradient

(inkscape:76406): Gtk-WARNING **: Failed to set text from markup due to error parsing markup: Error on line 1 char 71: Invalid UTF-8 encoded text in name - not valid 'UH\x89\xe5H\x8d=\x8b1 selected out of 64 mesh handles on 1 selected object'

(inkscape:76406): Gtk-WARNING **: Failed to set text from markup due to error parsing markup: Error on line 1 char 71: Invalid UTF-8 encoded text in name - not valid 'UH\x89\xe5H\x8d=\x8b1 selected out of 64 mesh handles on 1 selected object'
Assertion failed: (drag_valid), function update_handles, file ../../src/sp-mesh-array.cpp, line 1961.

Program received signal SIGABRT, Aborted.
0x00007fff93822ce2 in __pthread_kill ()
(gdb)

Confirmed with r12682 on OS X 10.7.5

Based on tests with archived builds:
- not reproduced with rev <= 12531
- reproduced with rev >= 12532
this regression was introduced with the merge of the C++ification branch in revision 12532:
<http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/revision/12532>

(Note: the markup warnings are not related to the regression - they are present ever since the mesh tool was merged into trunk with builds complied with llvm-gcc-4.2 on OS X 10.7.5 - FSF GCC and clang builds don't produce them).

[Markus Engel (engelmarkus)]

The attached patch fixes this; note that the code has only ever worked because the wrong constant was used in the type check. I'm not sure now what all these other functions are good for.

[su_v (suv-lp)]

Patch tested successfully with r12701 on OS X 10.7.5: with the patch applied, more than one stop can be assigned a different color, and dragging a gradient stop no longer produces a crash.

[Markus Engel (engelmarkus)]

Fixed in r12703.

[Tavmjong Bah (tavmjong-free)]

This fix broke undo for meshes. When 'undo' is invoked by the user, the XML tree is reverted to a previous state. This needs to force the mesh array to be rebuilt or the display is not updated.