[read-fonts] Only inspect reachable lookups for closure

Previously we just looked at every lookup in the lookup list when
computing the closure. This sounds okay at first glance, but it
overlooks the fact that the lookup list can include lookups that are
only reachable via other contextual lookups.

This introduces a much more rigorous approach, where on each run we
recalculate the set of reachable lookups, by checking to see if the
current set of glyphs matches any (chain) contextual rules, and then
adding the appropriate lookups from those rules.

font-test-data/src/lib.rs

@@ -53,6 +53,13 @@ pub mod closure {
     pub static RECURSIVE: &[u8] = include_bytes!("../test_data/ttf/recursive_closure.ttf");
     pub static RECURSIVE_GLYPHS: &str =
         include_str!("../test_data/fea/recursive_closure_glyphs.txt");
+    pub static CONTEXTUAL: &[u8] = include_bytes!("../test_data/ttf/context_closure.ttf");
+    pub static CONTEXTUAL_GLYPHS: &str =
+        include_str!("../test_data/fea/context_closure_glyphs.txt");
+    pub static RECURSIVE_CONTEXTUAL: &[u8] =
+        include_bytes!("../test_data/ttf/recursive_context_closure.ttf");
+    pub static RECURSIVE_CONTEXTUAL_GLYPHS: &str =
+        include_str!("../test_data/fea/recursive_context_closure_glyphs.txt");
 }
 
 pub mod post {

font-test-data/test_data/fea/context_closure.fea

@@ -0,0 +1,28 @@
+feature SUB6 {
+    lookup GSUB6f1 {
+        sub one two three' four' five six seven by X;
+        sub two one three' four' six five seven by Y;
+    } GSUB6f1;
+
+    # format 2 is generally less efficient than format 3 and hard to generate
+    # (as in, I'm not aware of any input that will compile to format 2)
+    lookup GSUB6f3 {
+        sub [space comma semicolon] e' by e.2;
+    } GSUB6f3;
+} SUB6;
+
+# can't declare in a feature or it gets added to the feature
+lookup MY_RULES {
+    sub f by f.2;
+} MY_RULES;
+
+feature SUB5 {
+    lookup GSUB5f1 {
+        sub a' b' by a_b;
+        sub c' d' by c_d;
+    } GSUB5f1;
+
+    lookup GSUB5f3 {
+       sub f' lookup MY_RULES g';
+    } GSUB5f3;
+} SUB5;

font-test-data/test_data/fea/context_closure_glyphs.txt

@@ -0,0 +1,24 @@
+.notdef
+space
+comma
+semicolon
+one
+two
+three
+four
+five
+six
+seven
+X
+Y
+a
+b
+c
+d
+e
+f
+g
+e.2
+f.2
+a_b
+c_d

font-test-data/test_data/fea/recursive_context_closure.fea

@@ -0,0 +1,12 @@
+feature test {
+
+    lookup first {
+        sub a b' c by B;
+        sub a B.2' by B.3;
+    } first;
+
+    lookup second {
+        sub a B' c by B.2;
+    } second;
+
+} test;

font-test-data/test_data/fea/recursive_context_closure_glyphs.txt

@@ -0,0 +1,6 @@
+a
+b
+c
+B
+B.2
+B.3