Rendition width and height fields should allow non-int numbers for SVG

[jams2]

Rendition queries may operate on SVGs, for which non-integer width and height attributes are valid. The type for these fields is currently defined as Int:

wagtail-grapple/grapple/types/images.py

Line 88 in 86f0357

width = graphene.Int(required=True)

References:

• https://www.w3.org/TR/SVG11/struct.html#SVGElementWidthAttribute
• https://www.w3.org/TR/SVG11/types.html#DataTypeLength
• https://www.w3.org/TR/SVG11/types.html#DataTypeNumber

This can cause an error: Int cannot represent non-integer value: 342.5