GL3Plus: If GL_ARB_copy_image is not available, use GL_NV_copy_image

[traversaro]

This fixes crashes like:

terminate called after throwing an instance of 'Ogre::UnimplementedException'
  what():  OGRE EXCEPTION(9:UnimplementedException):  in GL3PlusTextureGpu::copyTo at /build/ogre-next-UFfg83/ogre-next-2.2.5+dfsg3/RenderSystems/GL3Plus/src/OgreGL3PlusTextureGpu.cpp (line 677)
Stack trace (most recent call last) in thread 3740:
#10   Object "[0xffffffffffffffff]", at 0xffffffffffffffff, in
#9    Object "/lib/x86_64-linux-gnu/libc.so.6", at 0x7f5543e699ff, in
#8    Object "/lib/x86_64-linux-gnu/libc.so.6", at 0x7f5543dd7b42, in
#7    Object "/lib/x86_64-linux-gnu/libQt5Core.so.5", at 0x7f553e22399d, in
#6    Object "/lib/x86_64-linux-gnu/libQt5Core.so.5", at 0x7f553e221f90, in qTerminate()
#5    Object "/lib/x86_64-linux-gnu/libstdc++.so.6", at 0x7f553fe642f6, in std::terminate()
#4    Object "/lib/x86_64-linux-gnu/libstdc++.so.6", at 0x7f553fe6428b, in
#3    Object "/lib/x86_64-linux-gnu/libstdc++.so.6", at 0x7f553fe58bfd, in
#2    Object "/lib/x86_64-linux-gnu/libc.so.6", at 0x7f5543d6b7f2, in abort
#1    Object "/lib/x86_64-linux-gnu/libc.so.6", at 0x7f5543d85475, in raise
#0    Object "/lib/x86_64-linux-gnu/libc.so.6", at 0x7f5543dd9a7c, in pthread_kill
Aborted (Signal sent by tkill() 3665 1000)

when OgreNext runs on system that do not implement GL_ARB_copy_image, such as d3d12 mesa driver, that is used for hardware acceleration on WSLg (see gazebosim/gz-sim#920).

The original suggestion for this fix comes from @bperseghetti and @PetervdPerk-NXP, see gazebosim/gz-sim#920 (comment) .