Dropping const qualifier in IGraphics/Platforms/IGraphicsWin

[MeijisIrlnd]

Description

(I feel weird opening an issue about this because it's such a tiny issue, but was going to open a PR and it told me to, so sorry if this is overkill)
Under MSVC and C++20, compilation (all targets) of Examples/IPlugEffect fails with

Conversion from string literal loses const qualifier (see /Zc:strictStrings)

on the declaration of wndClassName. I assume something must have changed with assigning string literals to pointers in c++20 or something along those lines (?)

To Reproduce

I encountered it using the recently-rebased cmake branch, building IPlugEffect, and changing the c++ standard from 17 to 20 in the CMakeLists:

iplug_target_add(_base INTERFACE
  INCLUDE ${PROJECT_DIR} ${PROJECT_DIR}/resources
  LINK iPlug2_NanoVG_GL2
  FEATURE cxx_std_20)

Other than this, C++20 with MSVC seems to work out the box!