Link to PDF file (or attach file here):
https://issues.apache.org/jira/secure/attachment/12650283/000004_ShadingPatternTextPDF.pdf

Configuration:
- Web browser and its version: FF 51.0.1 32bit
- Operating system and its version: W7
- PDF.js version: latest

Steps to reproduce the problem:
display file

What is the expected behavior? (add screenshot)
![ok](https://cloud.githubusercontent.com/assets/6665575/23339534/5ff2de9c-fc25-11e6-837e-0fdaf81b4ca9.png)

What went wrong? (add screenshot)
![nok](https://cloud.githubusercontent.com/assets/6665575/23339536/63f7a37e-fc25-11e6-813c-cc218cc4def3.png)

possibly similar problem:
https://www.pdflib.com/fileadmin/pdflib/Cookbook/pdf/color_gradient.pdf (shaded text completely missing)
https://issues.apache.org/jira/secure/attachment/12630566/pslib-shading.pdf (page 4, part of text missing)
https://issues.apache.org/jira/secure/attachment/12652009/pattern-shading-2-4-idMatrix.pdf (some text missing)
https://issues.apache.org/jira/secure/attachment/12652010/pattern-shading-2-4-noMatrix.pdf (some text missing)

operatorList,
    executionStartIdx,
    continueCallback,
    stepper
  ) {
    const argsArray = operatorList.argsArray;
    const fnArray = operatorList.fnArray;
    let i = executionStartIdx || 0;
    const argsArrayLen = argsArray.length;
    
    if (argsArrayLen === i) {
      return i;
    }
    const chunkOperations =
      argsArrayLen - i > EXECUTION_STEPS &&
      typeof continueCallback === "function";
    const endTime = chunkOperations ? Date.now() + EXECUTION_TIME : 0;
    let steps = 0;
    const commonObjs = this.commonObjs;
    const objs = this.objs;
    let fnId;
    while (true) {
      if (stepper !== undefined && i === stepper.nextBreakPoint) {
        stepper.breakIt(i, continueCallback);
        return i;
      }
      fnId = fnArray[i];
      if (fnId !== OPS.dependency) {
        this[fnId].apply(this, argsArray[i]);