Fix image plane to mesh conversion

[ravencgg]

Fixes a crash in the memcpy on non-square images. Fixes the orientation and placement of the mesh.

[guillaumechereau]

Hello, what bug is this fixing exactly? I tried the patch and turning an image layer into a mesh now seems to revert the image.

[guillaumechereau]

Oh I see, there is indeed a memory bug. Your patch fixes it, but now the mesh is flipped somehow.

[ravencgg]

You're right, my test image was symmetrical on the x axis so I missed it. Fixed in latest commit.

[guillaumechereau]

Thanks!