Simply the matrix calculation for zooming

Simply the matrix calculation for zooming

In order to apply the zoom transformation to the output matrix, Weston was
doing the following:

• Create a temporary matrix to hold the translation
• Invert the translation matrix using weston_matrix_invert into
  another temporary matrix
• Scale that matrix by the scale factor
• Multiply the current matrix with the temporary matrix

Using weston_matrix_invert to invert a translation matrix is over the top.
Instead we can just negate the values we pass to weston_matrix_translate.
Matrix multiplication is associative so creating a temporary matrix to hold the
scale and translation transform should be equivalent to just applying them
directly to the output matrix.