FTMatrix.Multiply(mrot, mrot2) static method does not work correctly.

[HinTak]

see https://github.com/Robmaister/SharpFont/issues/86#issuecomment-231745179 for example code to demo this problem.

[HinTak]

Okay, after the change, this is more sane:

Mono C# Shell, type "help;" for help

Enter statements below.
csharp> using SharpFont;
csharp> var a = new FTVector(new Fixed16Dot16(1), new Fixed16Dot16(0));
csharp> a.Rotate(new Fixed16Dot16(30));
csharp> a.X
0.86602783203125
csharp> a.Y
0.5
csharp> var b = new FTVector(new Fixed16Dot16(1), new Fixed16Dot16(0));
csharp> b.Rotate(new Fixed16Dot16(30 + 90));
csharp>  FTMatrix mrot = new FTMatrix(a, b);
csharp> FTMatrix mrot2 = new FTMatrix(a, b);
csharp> FTMatrix mrot3 = new FTMatrix(a, b);
csharp> FTMatrix.Multiply(ref mrot, ref mrot2)
csharp> mrot2.XX
0.5
csharp> mrot2.XY 
0.86602783203125
csharp> mrot2.YX 
-0.86602783203125
csharp> mrot2.YY 
0.5
csharp> mrot3.XX 
0.86602783203125
csharp> mrot3.XY 
0.5
csharp> mrot3.YX 
-0.5
csharp> mrot3.YY 
0.86602783203125
csharp> mrot.XX  
0.86602783203125
csharp> mrot.XY 
0.5
csharp> mrot.YX 
-0.5
csharp> mrot.YY 
0.86602783203125
csharp> FTMatrix.Multiply(ref mrot, ref mrot2) 
csharp> mrot2.XX                               
0
csharp> mrot2.XY 
1
csharp> mrot2.YX 
-1
csharp> mrot2.YY 
0
csharp> FTMatrix.Multiply(ref mrot, ref mrot2) 
csharp> mrot2.XX                               
-0.5
csharp> mrot2.XY 
0.86602783203125
csharp> mrot2.YX 
-0.86602783203125
csharp> mrot2.YY 
-0.5
csharp>  

[Robmaister]

Since the PropertyGet/PropertySet stuff was fixed in a different way, can you open a new PR with just these two changes (this + #87) rebased onto the current master?

I would also prefer if the old signatures stayed but had an ObsoleteAttribute applied to maintain API compatibility (even though they're broken) until the next major version bump. I did the same thing with the PropertyGet overloads with redundant out modifier for a class property.