Fix ROOT-5287, by calling InitializeGraphics in TCanvas::Init

gApplication->InitializeGraphics must be called after loading the Gpad library
(but *not* during the loading itself, as it need the dictionary to be fully
loaded).  Previously it was called from gSystem->Load.  Before this commit, it
was not called when the library is loaded in reaction to a missing symbol, i.e.
at 'JIT link time', by NotifyLazyFunctionCreators which calls TCling's
autoloadCallback which does *not* call InitializeGraphics.

Adding InitializeGraphics to TCling's autoloadCallback is not a viable solution
since it is being called during the 'link-phase' of the code generation (aka at
the end of llvm::JIT::getPointerToFunction) and because the JIT is not
re-entrant (it has an explicit assert against it: "Error: Recursive compilation
detected!") *and* because InitializeGraphics must load the plugin scripts ...
which means compiling.

So instead we delay the call to InitializeGraphics until TCanvas::init (and if
find other places that needs it, we will put it there too).