plot: make GlyphBox support plot area-dependent sized glyphs

At the moment, a plot.GlyphBox represents a zero-sized point in the data space surrounded by an pair of offsets in the plot space. There is no way to represent the extent of glyphs that have a size dependent on the size of the plot.

This is due to the fact that the GlyphBoxes method is not passed any information regarding the plot area.

This can be fixed by adding a second pair of normalised x,y coordinates.

```
type GlyphBox struct {
// The glyph location in normalized coordinates.
MinX, MinY float64
MaxX, MaxY float64

// Rect is the offset of the glyph's minimum drawing
// point relative to the glyph location and its size.
Rect
}
```

The details of types etc need to be fleshed out.