Keep processing Graphics if there are pending operations

[darksylinc]

Fixes #90017
Fixes #90030
Fixes #98044

This PR makes the following changes:

Force processing of GPU commands for frame_count frames

The variable frames_pending_resources_for_processing is added to track this.

The ticket #98044 suggested to use _flush_and_stall_for_all_frames() while minimized.

Technically this works and is a viable solution.

However I noticed that this issue was happening because Logic/Physics continue to work "business as usual" while minimized(*). Only Graphics was being deactivated (which caused commands to accumulate until window is restored).

To continue this behavior of "business as usual", I decided that GPU work should also "continue as usual" by buffering commands in a double or triple buffer scheme until all commands are done processing (if they ever stop coming). This is specially important if the app specifically intends to keep processing while minimized.

Calling _flush_and_stall_for_all_frames() would fix the leak, but it would make Godot's behavior different while minimized vs while the window is presenting.

(*) OS::add_frame_delay does consider being minimized, but it just throttles CPU usage. Some platforms such as Android completely disable processing because the higher level code stops being called when the app goes into background. But this seems like an implementation-detail that diverges from the rest of the platforms (e.g. Windows, Linux & macOS continue to process while minimized).

Rename p_swap_buffers for p_present

This is potentially a breaking change (if it actually breaks anything, I ignore. But I strongly suspect it doesn't break anything).

"Swap Buffers" is a concept carried from OpenGL, where a frame is "done" when glSwapBuffers() is called, which basically means "present to the screen".

However it also means that OpenGL internally swaps its internal buffers in a double/triple buffer scheme (in Vulkan, we do that ourselves and is tracked by RenderingDevice::frame).

Modern APIs like Vulkan differentiate between "submitting GPU work" and "presenting".

Before this PR, calling RendererCompositorRD::end_frame(false) would literally do nothing. This is often undesired and the cause of the leak. After this PR, calling RendererCompositorRD::end_frame(false) will now process commands, swap our internal buffers in a double/triple buffer scheme but avoid presenting to the screen.

Hence the rename of the variable from p_swap_buffers to p_present (which slightly alters its behavior).
If we want RendererCompositorRD::end_frame(false) to do nothing, then we should not call it at all.

This PR reflects such change: When we're minimized and has_pending_resources_for_processing() returns false, we don't call RendererCompositorRD::end_frame() at all.

But if has_pending_resources_for_processing() returns true, we will call it, but with p_present = false because we're minimized.

There's still the issue that Godot keeps processing work (logic, scripts, physics) while minimized, which we shouldn't do by default. But that's work for follow up PR.

[clayjohn]

Looks great!

There are two places where we call draw(false). Both are in editor classes (resource preview and onion skinning). In both cases we are just forcing the rendering server to redraw a viewport without needing to wait for vsync and draw to the main screen. So I don't think either one will regress with this change. In fact, this might be an improvement in both cases as it seems like they wouldn't properly end the frame previously.

[Repiteo]

Thanks!