dcs: sort of implement DCS passthrough

We now store the passthrough characters in a buffer, and call
dcs_passthrough() on unhook.

However, dcs_passthrough() doesn't do anything.

dcs.c

@@ -0,0 +1,32 @@
+#include "dcs.h"
+
+#define LOG_MODULE "dcs"
+#define LOG_ENABLE_DBG 0
+#include "log.h"
+
+void
+dcs_passthrough(struct terminal *term)
+{
+    LOG_DBG("DCS passthrough: %.*s (%zu bytes)",
+            (int)term->vt.dcs.idx, term->vt.dcs.data, term->vt.dcs.idx);
+}
+
+bool
+dcs_ensure_size(struct terminal *term, size_t required_size)
+{
+    if (required_size <= term->vt.dcs.size)
+        return true;
+
+    size_t new_size = (required_size + 127) / 128 * 128;
+    assert(new_size > 0);
+
+    uint8_t *new_data = realloc(term->vt.dcs.data, new_size);
+    if (new_data == NULL) {
+        LOG_ERRNO("failed to increase size of DCS buffer");
+        return false;
+    }
+
+    term->vt.dcs.data = new_data;
+    term->vt.dcs.size = new_size;
+    return true;
+}