Method to render a formula to an existing bitmap

### Motivation
To avoid reallocations for every created bitmap. Useful, when rendering a formula for the same control/file.

### Details
Proposed syntax for `TexRenderer`:
```cs
public void RenderToBitmap(RenderTargetBitmap dst, double x, double y)
```

The idea is to render it to the given bitmap of type `RenderTargetBitmap`.
Because the rendered image has an unpredictable size, I suggest resizing it internally to the size of the destination bitmap.