[css-paint-api] Specify explicitly that PaintSize.{width, height} may be fractional

[chrishtr]

The paint API specifies that the width and height fields of PaintInfo are doubles, but
elsewhere in the spec (https://drafts.css-houdini.org/css-paint-api/#2d-rendering-context)
says to "Set bitmap dimensions for the context’s output bitmap to width and height", which
implies that the width and height are integers.

Instead, I suggest that phrase should be changed to "Set bitmap dimensions for the context’s output bitmap to the rounded values of width and height".

(Note: the actual device-pixel backing buffer may have a different size than the output bitmap. This
is accounted for by devicePixelRatio.)

[chrishtr]

Also, an un-rounded width and height plus devicePixelRatio and specifying the top-left
of the painting canvas to be positioned at the device pixel location of the top-left of where
the element will actually be drawn to the screen, allows the developer, if they wish, to do pixel-perfect painting.

This is why it's important for width and height to be doubles.

[bfgeek]

This has been fixed by PR #500