Add Texture2D.GetDataPointerEXT

[kg]

Without this, it's not possible to cleanly queue up texture readback operations to execute later, since GetData checks the size of T against the texture format, instead of checking the size of the buffer - so you can't read a Color texture into a byte[] even if it's big enough.

[flibitijibibo]

I might be misremembering but I thought this worked like SetData where byte[] was okay... are we just missing some math checks that exist in SetData?

[kg]

I might be misremembering but I thought this worked like SetData where byte[] was okay... are we just missing some math checks that exist in SetData?

The

	int elementSizeInBytes = MarshalHelper.SizeOf<T>();
	ValidateGetDataFormat(Format, elementSizeInBytes);

should reject an attempt to download a Color surface into a byte[], I think. IIRC the last time I tried it, it didn't work.