_clipImageData is GC expensive

[Tyriar]

Part of microsoft/vscode#161622

During vscode startup:

[Tyriar]

This was when connecting to existing terminals with existing content which is hard to reproduce on the demo. A close approximation is the load test button, see the memory chart:

[Tyriar]

Actually this might be an issue with the whole draw to cache call, closing for not until I understand the problem better and whether it's worth fixing.

[jerch]

Looking at the code of that function, I think you can spare the new Uint8ClampedArray there by directly creating the ImageDate and writing to its .data? Would atleast lower the gc pressure somewhat.

The bytes also could be transferred as uint32, would make the function somewhat faster.

[Tyriar]

Great idea, then it's just a new typed array view on the same buffer until we eventually throw it away after it gets drawn to the canvas!