Visualization, Path Tracing - Improve interactivity in "steady" rendering mode

In similar issue (28114, Visualization, Ray tracing - Make ray tracing mode interactive in high resolutions) an attempt has been made to improve interactivity during interaction with the scene. However, when the camera is stabilized the performance drops significantly (to its "normal" level).

However, it is possible to prevent this drop also by improving adaptive screen sampling mode. The important feature of this mode is that it is possible to keep the same number of tiles for any screen resolution (e.g. 256 tiles can be used for both 512 x 512 window and 1920 x 1080 window). So, a smaller number of tiles allows to increase interactivity (FPS), but at the cost of higher per-frame variance ('noise'). On the contrary a larger number of tiles decrease interactivity, but leads to lower per-frame variance. Note that the total time needed to produce final final image is the same for both cases. In frames of this issue, it is proposed to keep number of tiles quite low (e.g. 256) independently on the window size. This will allow:

1) Keep interactivy at a high level (FPS) which is especially important for low-performance GPUs.

2) Prevent freezing of application GUI and possible crashes due to driver's limit on maximum frame rendering time (it is related to complex scenes).