This change calculates the face-normal of the triangle by adding
the three vertex-normals and then normalizing. This results in an
average of the three vertex-normals.
glTexCoord should behave like glColor. It only updates a gl context
variable that contains the current texture coordinates. The vertex is
only actually created once glVertex is called.
Models that contain UV co-ordinates are now supported,
and will display with a texture wrapped around it, provided
a `bmp` with the same name as the object is in the same
directory as the 3D Model.
