Fix print problems with HL-L2320D and others

- reduce max_lines_per_block from 64 to 32 in block.h, which seems
to improve print success rate

src/block.h

@@ -59,7 +59,7 @@ class block {
 
  private:
   static const unsigned max_block_size_ = 16350;
-  static const unsigned max_lines_per_block_ = 64;
+  static const unsigned max_lines_per_block_ = 32;
 
   std::vector<std::vector<uint8_t>> lines_;
   int line_bytes_;

test/test_block.cc

@@ -40,7 +40,7 @@ const lest::test specification[] = {
   "A block can contain 64 lines",
   [] {
     block b;
-    for (int i = 0; i < 64; ++i) {
+    for (int i = 0; i < 32; ++i) {
       EXPECT(b.line_fits(1));
       b.add_line(vec(1));
     }