internal/unsafe: fix SliceOf to return len equals to cap

Broken by gioui.org/commit/2dce8a0155ae192f54f74747d660b9f311e9e364,
found while debugging the dormant OpenGL backend for Windows.

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

internal/unsafe/unsafe.go

@@ -41,6 +41,7 @@ func SliceOf(s uintptr) []byte {
 	h := (*reflect.SliceHeader)(unsafe.Pointer(&res))
 	h.Data = s
 	h.Cap = 1 << 30
+	h.Len = 1 << 30
 	return res
 }