Opentype parsing, layout and shaping will be used by subpackages to package
font. Move the opentype package accordingly.
Remove the Must helper function; programs will no longer use the opentype
package in the normal case.
Signed-off-by: Elias Naur <mail@eliasnaur.com>
Package font holds a singleton text.Shaper for general use. Subpackages
call Register to add fonts to the registry.
The intention is that programs can add a typeface by importing a package:
import _ "gioui.org/font/gofont"
Signed-off-by: Elias Naur <mail@eliasnaur.com>