GlContext::createSwapChain fails when g_platformData.context != nullptr

[AdamBalazsVay]

Steps to reproduce the behavior:

1. Init BGFX with init.platformData.context != nullptr
2. In GlContext::create m_display won't be set, remains nullptr:

   bgfx/src/glcontext_egl.cpp

   Line 283 in 336cb9f

   m_display = eglGetDisplay(NULL == ndt ? EGL_DEFAULT_DISPLAY : ndt);

3. In GlContext::create m_config won't be set:

   bgfx/src/glcontext_egl.cpp

   Line 356 in 336cb9f

   success = eglChooseConfig(m_display, attrs, &m_config, 1, &numConfig);

4. GlContext::createSwapChain fails, as m_display and m_config are invalid:

   bgfx/src/glcontext_egl.cpp

   Line 582 in 336cb9f

   return BX_NEW(g_allocator, SwapChainGL)(m_display, m_config, m_context, (EGLNativeWindowType)_nwh, _width, _height);

Expected behavior
BGFX should work with an externally set context, GlContext's m_display and m_config should contain meaningful data even in these cases.