text,widget: [API] drop runereader based shaping API

The io.Reader based API has the potential to be significantly more
efficient, and there are very few users of the runereader API. This
commit simply drops it entirely in favor of the reader API.

Signed-off-by: Chris Waldon <christopher.waldon.dev@gmail.com>

text/shaper_test.go

@@ -113,7 +113,7 @@ func TestShapingNewlineHandling(t *testing.T) {
 			}
 			checkGlyphs()
 
-			cache.LayoutReader(Parameters{
+			cache.Layout(Parameters{
 				Alignment: Middle,
 				PxPerEm:   fixed.I(10),
 			}, 200, 200, english, strings.NewReader(tc.textInput))