Fixed signed/unsigned mismatch

Change-Id: Ibd7a099fa0a25dbddb1733b81101abc82ddd3f4c
Reviewed-on: https://swiftshader-review.googlesource.com/14008
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/src/OpenGL/libGLESv2/Context.cpp b/src/OpenGL/libGLESv2/Context.cpp
index 33ac1d9..8a1a4b7 100644
--- a/src/OpenGL/libGLESv2/Context.cpp
+++ b/src/OpenGL/libGLESv2/Context.cpp
@@ -1596,7 +1596,7 @@
 		if(mState.pixelUnpackBuffer->name)
 		{
 			if(mState.pixelUnpackBuffer->isMapped() ||
-			   (mState.pixelUnpackBuffer->size() < imageSize) ||
+			   (mState.pixelUnpackBuffer->size() < static_cast<size_t>(imageSize)) ||
 			   ((*data) && (imageSize % static_cast<GLsizei>((ptrdiff_t)(*data)))))
 			{
 				return GL_INVALID_OPERATION;