Fix EnsoGL runtime statistics collection and processing, so that all values in a single vertical slice of the Monitor Panel graph (accessible through `control-option-tilde` shortcut) represent stats for the same animation/rendering frame.
Before the change, for any vertical slice of the graph, the FPS value was shown for the previous (`i - 1`) rendering frame, whereas the other values were shown for the current (`i`) rendering frame.
The alignment of the stats will become even more important in the future, when the values will be included in more detailed profiling reports, which is planned.
The GUI that is impacted (if in a subtle way) by the changed code is the performance monitor panel, accessible with a `ctrl-option-tilde` keyboard shortcut in the IDE.
https://www.pivotaltracker.com/story/show/181140499
The goal of this change is to decouple the code calculating EnsoGL runtime stats (notably, FPS, frame time, and WASM memory usage) from `Sampler` trait implementations (this trait is intended only to be responsible for post-processing stats data for display/view purposes).
Apart from the general improvement in separation of responsibilities, a longer-term goal for this change is to make it possible to access all the stats in any code that doesn't need the `Sampler` functionalities (expected to be needed for https://www.pivotaltracker.com/story/show/181093832 and https://www.pivotaltracker.com/story/show/181093601).
The main loop's code is performance-critical, and this has implications
on tasks design and planning. Knowing this earlier would help me avoid
some major workflow inefficiencies while working on tasks touching this
code.
