app: fix gimp_transform_matrix_generic()
When the resulting matrix transforms all input points behind the camera, negate the matrix, instead of failing, which results in a matrix that transforms the input points to the corresponding points in front of the camera. This avoids rejecting certain valid transforms as invalid, in the generic transform tools (unified, perspective, and handle transform). Make the number of input and output points explicit in the function's signature, and add comments.
Showing with 54 additions and 10 deletions