Fix int32 overflow bugs with deep images

A 32k x 32k image * 8 byte pointers leads to an overflow when trying
to find the sample count of a deep pixel.

Signed-off-by: Larry Gritz <lg@imageworks.com>

OpenEXR/IlmImf/ImfMisc.h

@@ -99,7 +99,7 @@ inline
 int&
 sampleCount(char* base, int xStride, int yStride, int x, int y)
 {
-    char* ptr = base + y * yStride + x * xStride;
+    char* ptr = base + y * ptrdiff_t(yStride) + x * ptrdiff_t(xStride);
     int* intPtr = (int*) ptr;
 
     return *intPtr;
@@ -110,7 +110,7 @@ inline
 const int&
 sampleCount(const char* base, int xStride, int yStride, int x, int y)
 {
-    const char* ptr = base + y * yStride + x * xStride;
+    const char* ptr = base + y * ptrdiff_t(yStride) + x * ptrdiff_t(xStride);
     int* intPtr = (int*) ptr;
 
     return *intPtr;