affine2d - 2D affine transforms

`local matrix = require'affine2d'`

`matrix(xx, yx, xy, yy, x0, y0) -> mt` `matrix() -> mt`	Create a new matrix object. Defaults to identity matrix `1, 0, 0, 1, 0, 0`.
`mt:set(xx, yx, xy, yy, x0, y0) -> mt`	Set the matrix fields.
`mt:reset() -> mt`	Set the matrix to the identity matrix `1, 0, 0, 1, 0, 0`.
`mt:unpack() -> xx, yx, xy, yy, x0, y0`	Unpack the matrix fields.
`mt:copy() -> newmt`	Create a new matrix object with the same fields as `mt`.
`mt:transform_point(x, y) -> tx, ty` `mt(x, y) -> tx, ty`	Transform a 2D point.
`mt:transform_distance(x, y) -> dx, dy`	Transform a point ignoring translation.
`mt:multiply(bxx, byx, bxy, byy, bx0, by0) -> mt`	Multiply `mt * b` and store the result in `mt`.
`mt:transform(bxx, byx, bxy, byy, bx0, by0) -> mt`	Multiply `b * mt` and store the result in `mt`.
`mt:determinant() -> det`	Compute the matrix determinant.
`mt:is_invertible() -> true \| false`	Check if the matrix is invertible, that is, if the determinant is not 0, 1/0 or -1/0.
`mt:scalar_multilpy(s) -> mt`	Mulitply each field of the matrix with a scalar value.
`mt:inverse() -> newmt`	Return the inverse matrix, or nothing if the matrix is not invertible.
`mt:translate(x, y) -> mt`	Translate the matrix.
`mt:scale(sx, sy) -> mt` `mt:scale(s)`	Scale the matrix.
`mt:rotate(angle) -> mt`	Rotate the matrix. The angle is in degrees.
`mt:skew(angle_x, angle_y) -> mt`	Skew the matrix. Angles are in degrees.
`mt:is_identity() -> true \| false`	Check if the matrix is the identity matrix, thus having no effect on the input.
`mt:has_unity_scale() -> true \| false`	Check that no scaling is done with this transform, only flipping and multiple-of-90-degree rotation.
`mt:has_uniform_scale() -> true \| false`	Check that scaling with this transform is uniform on both axes.
`mt:scale_factor() -> s`	Largest dimension of the bounding box of the transformed unit square.
`mt:is_pixel_exact() -> true \| false`	Check that pixels map 1:1 with this transform so that no filtering is necessary to project an image to the screen for example.
`mt:is_straight() -> true \| false`	Check that there's no skew and that there's no rotation other than multiple-of-90-degree rotation.
`mt1 * mt2 -> newmt`	Multiply two matrices and return the result as a new matrix.
`mt1 == mt2`	Test two matrices for equality. Matrices are considered equal when their fields are equal.