code: func (c *Canvas) SetMatrix(m pixel.Matrix) {
  	// pixel.Matrix is 3x2 with an implicit 0, 0, 1 row after it. So
  	// [0] [2] [4]    [0] [3] [6]
  	// [1] [3] [5] => [1] [4] [7]
  	//  0   0   1      0   0   1
  	// since all matrix ops are affine, the last row never changes, and we don't need to copy it
  	for i, j := range [...]int{0, 1, 3, 4, 6, 7} {
  		c.mat[j] = float32(m[i])
  	}
  }
docs: // SetMatrix sets a Matrix that every point will be projected by.