gpu: support forced OpenGL ES mode, for ANGLE on macOS

macOS is the only platform where desktop OpenGL is used. To support
foreign ANGLE contexts add a setting to override Gio's selection of
OpenGL implementation.

The bulk of this change is making all function pointers per-context
instead of global, and loading the OpenGL library dynamically. As a side
effect we're closer to Gio tolerating a platform without any OpenGL
implementation. For example, Apple has deprecated OpenGL and OpenGL ES
on its platforms and may remove them in the future.

Note that as a side-effect of this change, Gio needs Go 1.16 or newer to
run on iOS.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

internal/gl/gl_windows.go

@@ -98,7 +98,7 @@ type Functions struct {
 
 type Context interface{}
 
-func NewFunctions(ctx Context) (*Functions, error) {
+func NewFunctions(ctx Context, forceES bool) (*Functions, error) {
 	if ctx != nil {
 		panic("non-nil context")
 	}