change API contract: disallow zero-sized images or surfaces

src/image/SkSurface.cpp

Index: src/image/SkSurface.cpp
diff --git a/src/image/SkSurface.cpp b/src/image/SkSurface.cpp
index dca4ce771c923047c270f057177a8edd49aa6db0..6fd40c10af809be81658f0983a7b1ba00eb2fbf3 100644
--- a/src/image/SkSurface.cpp
+++ b/src/image/SkSurface.cpp
@@ -123,16 +123,16 @@ static SkSurface_Base* asSB(SkSurface* surface) {
 SkSurface::SkSurface(int width, int height, const SkSurfaceProps* props)
     : fProps(SkSurfacePropsCopyOrDefault(props)), fWidth(width), fHeight(height)
 {
-    SkASSERT(fWidth >= 0);
-    SkASSERT(fHeight >= 0);
+    SkASSERT(fWidth > 0);
+    SkASSERT(fHeight > 0);
     fGenerationID = 0;
 }
 
 SkSurface::SkSurface(const SkImageInfo& info, const SkSurfaceProps* props)
     : fProps(SkSurfacePropsCopyOrDefault(props)), fWidth(info.width()), fHeight(info.height())
 {
-    SkASSERT(fWidth >= 0);
-    SkASSERT(fHeight >= 0);
+    SkASSERT(fWidth > 0);
+    SkASSERT(fHeight > 0);
     fGenerationID = 0;
 }