fix linux warnings

core/fxge/skia/fx_skia_device.cpp

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/fxge/include/fx_ge.h"
 
 #if defined(_SKIA_SUPPORT_)
 #include "core/fxcodec/include/fx_codec.h"
 
 #include "core/fpdfapi/fpdf_page/cpdf_shadingpattern.h"
@@ skipping 550 matching lines @@
       m_bRgbByteOrder(bRgbByteOrder),
       m_bGroupKnockout(bGroupKnockout) {
   SkBitmap skBitmap;
   SkImageInfo imageInfo =
       SkImageInfo::Make(pBitmap->GetWidth(), pBitmap->GetHeight(),
                         kN32_SkColorType, kOpaque_SkAlphaType);
   skBitmap.installPixels(imageInfo, pBitmap->GetBuffer(), pBitmap->GetPitch(),
                          nullptr, /* to do : set color table */
                          nullptr, nullptr);
   m_pCanvas = new SkCanvas(skBitmap);
+  if (m_bGroupKnockout)
+    SkDebugf("");  // FIXME(caryclark) suppress 'm_bGroupKnockout is unused'
 }
 
 CFX_SkiaDeviceDriver::CFX_SkiaDeviceDriver(int size_x, int size_y)
     : m_pBitmap(nullptr),
       m_pOriDevice(nullptr),
       m_pRecorder(new SkPictureRecorder),
       m_ditherBits(0),
       m_bRgbByteOrder(FALSE),
       m_bGroupKnockout(FALSE) {
   m_pRecorder->beginRecording(SkIntToScalar(size_x), SkIntToScalar(size_y));
@@ skipping 321 matching lines @@
     if (clipStart || clipEnd) {
       if (clipStart && start_r)
         skClip.addCircle(pts[0].fX, pts[0].fY, start_r);
       if (clipEnd)
         skClip.addCircle(pts[1].fX, pts[1].fY, end_r, SkPath::kCCW_Direction);
       else
         skClip.setFillType(SkPath::kInverseWinding_FillType);
       skClip.transform(skMatrix);
     }
     SkMatrix inverse;
-    skMatrix.invert(&inverse);
+    if (!skMatrix.invert(&inverse))
+      return false;
     skPath.addRect(skRect);
     skPath.transform(inverse);
   }
   m_pCanvas->save();
   if (!skClip.isEmpty())
     m_pCanvas->clipPath(skClip);
   m_pCanvas->concat(skMatrix);
   m_pCanvas->drawPath(skPath, paint);
   m_pCanvas->restore();
   return true;
@@ skipping 261 matching lines @@
   SetDeviceDriver(pDriver);
   return TRUE;
 }
 
 CFX_SkiaDevice::~CFX_SkiaDevice() {
   if (m_bOwnedBitmap && GetBitmap())
     delete GetBitmap();
 }
 
 #endif