Configuration:
- PDF.js version: 2.0.550

Steps to reproduce the problem:
1. Render document in SVG mode `renderMode="svg"`

What is the expected behavior? (add screenshot)
Rendering annotations same way as they rendered on canvas

What went wrong? (add screenshot)
```
Warning: Unimplemented operator beginAnnotations
Warning: Unimplemented operator endAnnotations
```
while (!found && pos > 0) {
        pos -= step - startXRefLength;
        if (pos < 0) {
          pos = 0;
        }
        stream.pos = pos;
        found = find(stream, STARTXREF_SIGNATURE, step, true);
      }
      if (found) {
        stream.skip(9);
        let ch;
        do {
          ch = stream.getByte();
        } while (isWhiteSpace(ch));
        let str = "";
        while (ch >=  0x20 && ch <=  0x39) {
          str += String.fromCharCode(ch);
          ch = stream.getByte();
        }
        startXRef = parseInt(str, 10);
        if (isNaN(startXRef)) {
          startXRef = 0;
        }
      }
    }
    return shadow(this, "startXRef", startXRef);
  }
  
  
  checkHeader() {