Fix memory leak in GraphicsContextState.

Source/platform/graphics/GraphicsContextState.cpp

Index: Source/platform/graphics/GraphicsContextState.cpp
diff --git a/Source/platform/graphics/GraphicsContextState.cpp b/Source/platform/graphics/GraphicsContextState.cpp
index 6ec377303f990cee13539c4002cf11c00ae85a2e..2084636f46e1684146fbb0a53dfc97447c05ee02 100644
--- a/Source/platform/graphics/GraphicsContextState.cpp
+++ b/Source/platform/graphics/GraphicsContextState.cpp
@@ -59,6 +59,7 @@ GraphicsContextState::GraphicsContextState(const GraphicsContextState& other)
 
 void GraphicsContextState::copy(const GraphicsContextState& source)
 {
+    this->~GraphicsContextState();

[jbroman, 2014/06/12 13:43:07]

Wouldn't it be more straightforward to just make this a member-wise copy (like
an ordinary operator= implementation) rather than destructing and reconstructing
it? I wouldn't expect that to be any slower.

From a simplicity perspective, it would be even nicer to destruct objects as we
stop using them (and so avoid leaking their references to shaders and whatnot
until a sufficiently deep state stack occurs again). That's the default
behaviour of a Vector, so I assume that we decided this was a net performance
win.

[Dominik Grewe, 2014/06/12 13:50:19]

We mainly did this to avoid code duplication. But I'm happy to change it back to
simple assignments.
I agree, it would be nicer. Not sure why we're doing it but, as you said, maybe
we decided it was faster this way.

     new (this) GraphicsContextState(source);
 }