text: consume io.Reader in shaper

io.Reader is actually a more efficient interface than io.RuneReader,
as we can pull bytes out and check for cache hits without doing
redundant rune<->string conversions. This isn't implemented yet,
however.

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.Layout(Parameters{
+			cache.LayoutReader(Parameters{
 				Alignment: Middle,
 				PxPerEm:   fixed.I(10),
 			}, 200, 200, english, strings.NewReader(tc.textInput))