Fix bad pointer arithmetic in SetBufferData (#33)

Fixes a bug where SetBufferData was writing garbage to the buffer if `startElement` was non-zero.

Rather than fixing the pointer addition (it should have been `ptr + startElement`) I opted to remove it and instead pass the index explicitly when grabbing the address of the array element. I think that's a little easier to understand, and it's slightly safer too -- if you pass a startElement beyond the array bounds it will now throw an IndexOutOfRangeException instead of silently reading from outside the array bounds.

Co-authored-by: Caleb Cornett <caleb.cornett@outlook.com>
Reviewed-on: #33
Co-authored-by: TheSpydog <thespydog@noreply.example.org>
Co-committed-by: TheSpydog <thespydog@noreply.example.org>

src/Graphics/CommandBuffer.cs

@@ -784,16 +784,14 @@ namespace MoonWorks.Graphics
 
 			var elementSize = sizeof(T);
 
-			fixed (T* ptr = &data[0])
+			fixed (T* ptr = &data[startElement])
 			{
-				var dataPtr = ptr + (startElement * elementSize);
-
 				Refresh.Refresh_SetBufferData(
 					Device.Handle,
 					Handle,
 					buffer.Handle,
 					bufferOffsetInBytes,
-					(IntPtr) dataPtr,
+					(IntPtr) ptr,
 					(uint) (numElements * elementSize)
 				);
 			}