Allow for a more modular renderer backends (#990)

* Draft GPU3D renderer modularization

* Update sources C++ standard to C++17

The top-level `CMakeLists.txt` is already using the C++17 standard.

* Move GLCompositor into class type

Some other misc fixes to push towards better modularity

* Make renderer-implementation types move-only

These types are going to be holding onto handles
of GPU-side resources and shouldn't ever be copied around.

* Fix OSX: Remove 'register' storage class specifier

`register` has been removed in C++17...
But this keyword hasn't done anything in years anyways.

OSX builds consider this "warning" an error and it
stops the whole build.

* Add RestartFrame to Renderer3D interface

* Move Accelerated property to Renderer3D interface

There are points in the code base where we do:
`renderer != 0` to know if we are feeding
an openGL renderer. Rather than that we can instead just have this be
a property of the renderer itself.
With this pattern a renderer can just say how it wants its data to come
in rather than have everyone know that they're talking to an OpenGL
renderer.

* Remove Accelerated flag from GPU

* Move 2D_Soft interface in separate header

Also make the current 2D engine an "owned" unique_ptr.

* Update alignment attribute to standard alignas

Uses standardized `alignas` rather than compiler-specific
attributes.

https://en.cppreference.com/w/cpp/language/alignas

* Fix Clang: alignas specifier

Alignment must be specified before the array to align the entire array.

https://en.cppreference.com/w/cpp/language/alignas

* Converted Renderer3D Accelerated to variable

This flag is checked a lot during scanline rasterization. So rather
than having an expensive vtable-lookup call during mainline rendering
code, it is now a public constant bool type that is written to only once
during Renderer3D initialization.

src/NDS.cpp

@@ -1564,7 +1564,7 @@ void RunTimer(u32 tid, s32 cycles)
 
 void RunTimers(u32 cpu)
 {
-    register u32 timermask = TimerCheckMask[cpu];
+    u32 timermask = TimerCheckMask[cpu];
     s32 cycles;
 
     if (cpu == 0)