fix crash when axes are rendered but m_bShowCubes is toggled off

[Pyrolistical]

m_bShowCubes can be toggled off by typing the letter c.
axes are rendered when a tracked device controller is connected.

when both m_bShowCubes is false and axes are rendered, the sample crashes.

this is because the g_MVPMatrix constant buffer is not being set for the axes rendering.

what was happening when m_bShowCubes was true, the axes rendering was inheriting the g_MVPMatrix constant buffer for the scene rendering.

this pr fixes the crash by always setting the g_MVPMatrix constant buffer for axes rendering.

[Pyrolistical]

the hellovr_vulkan also seems to have this bug, but I am not setup to test it.

this line should be copied to the axes rendering

openvr/samples/hellovr_vulkan/hellovr_vulkan_main.cpp

Line 3021 in ae46a8d

memcpy( m_pSceneConstantBufferData[ nEye ], GetCurrentViewProjectionMatrix( nEye ).get(), sizeof( Matrix4 ) );