Added PerspectiveTransform

[radarhere]

Furthers the work of #7684 to help clarify ImageTransform for #7683, by adding code and documentation for the missing ImageTransform - PerspectiveTransform.

This is based on AffineTransform, since it is similar.

Pillow/src/libImaging/Geometry.c

Lines 375 to 417 in a786a05

	static int
	affine_transform(double *xout, double *yout, int x, int y, void *data) {
	/* full moon tonight. your compiler will generate bogus code
	for simple expressions, unless you reorganize the code, or
	install Service Pack 3 */
	double *a = (double *)data;
	double a0 = a[0];
	double a1 = a[1];
	double a2 = a[2];
	double a3 = a[3];
	double a4 = a[4];
	double a5 = a[5];
	double xin = x + 0.5;
	double yin = y + 0.5;
	xout[0] = a0 * xin + a1 * yin + a2;
	yout[0] = a3 * xin + a4 * yin + a5;
	return 1;
	}
	static int
	perspective_transform(double *xout, double *yout, int x, int y, void *data) {
	double *a = (double *)data;
	double a0 = a[0];
	double a1 = a[1];
	double a2 = a[2];
	double a3 = a[3];
	double a4 = a[4];
	double a5 = a[5];
	double a6 = a[6];
	double a7 = a[7];
	double xin = x + 0.5;
	double yin = y + 0.5;
	xout[0] = (a0 * xin + a1 * yin + a2) / (a6 * xin + a7 * yin + 1);
	yout[0] = (a3 * xin + a4 * yin + a5) / (a6 * xin + a7 * yin + 1);
	return 1;
	}

If the 7th (g) and 8th (h) values are 0, then perspective_transform is identical to affine_transform, soPERSPECTIVE should be able to do everything that AFFINE can.

[hugovk]

Should we add to release notes?

[radarhere]

Ok, sure, I've added release notes.