Fix ZhangSuenSkeletonization error

[IsBrad]

Fix for bug https://github.com/accord-net/framework/issues/1267

From report:

The ZhangSuenSkeletonization class will cause a StackOverflowException when running on large(ish) images
This is due to a byte array almost the size of the input image being allocated on the stack.
.NET applications have a default stack size limit of 1mb for 32bit applications and 4mb for 64 bit.
This results in 1026x1026 & 2050x2050 almost exceeding the maximum stack size

The fix is to change this: (line 153)

var del0 = stackalloc byte[delSize]; for (var i = 0; i < delSize; i++) del0[i] = 0xFF;

To this:

var del0 = (byte*)Marshal.AllocHGlobal(delSize).ToPointer();

From testing this does not incur any measurable time penalty

[CLAassistant]

All committers have signed the CLA.

[fdncred]

Fixed with a previous merge.