Create VGL_WIDTH, VGL_HEIGHT, VGL_LENGTH for vglImage->shape.

The reason was to easy the job of understanding which constant
represented the pixel dimentions on the shape variable.
Refactored the image2d initialization using the new structure

some variables were replaced with new ones, this made me do some
alterations on the old image 2d methods.
Replaced all currencies of vglImage->width and vglImage->height for shape.