Renderers (Metal and OpenGL) should clip terminal if terminal grid size mismatches physical window

[mitchellh]

Both Metal and OpenGL have something like the following:

ghostty/src/renderer/OpenGL.zig

Lines 749 to 760 in 7eb35d7

	// If our terminal screen size doesn't match our expected renderer
	// size then we skip a frame. This can happen if the terminal state
	// is resized between when the renderer mailbox is drained and when
	// the state mutex is acquired inside this function.
	//
	// For some reason this doesn't seem to cause any significant issues
	// with flickering while resizing. '\_('-')_/'
	if (grid_size.rows != state.terminal.rows or
	grid_size.columns != state.terminal.cols)
	{
	return;
	}

In this case, we just stop rendering which makes it look like the terminal is hanging. We should clip the terminal area (if it's bigger than our window) or blank the padding (if it's smaller) in this scenario.

The only known way to trigger this is DECCOLM, so it's low priority since almost nothing uses this. But notably vtetest uses this and it makes it appear like we perform much worse than we do.