If you drag-select a slice of the profile off of the side of the
Profiler window, the profiler will try to render a negative start time,
which will overflow. This commit fixes that bug by clamping timestamps
to the start/end of the profile before rendering.
(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
Currently, there is no way to know when in a profile's duration a
sample was taken. This commit adds a basic timestamp to the timeline
widget, and a black bar to show where the cursor is hovering over.
