This displays statistics regarding frame timings and number of pixels
rendered.
Timings are based on the time between draw_debug_overlay() invocations.
This measures actual number of frames presented to the user vs. wall
clock time so this also includes everything the app might do besides
rendering.
Triangles are counted after clipping. This number might actually be
higher than the number of triangles coming from LibGL.
Pixels are counted after the initial scissor and coverage test. Pixels
rejected here are not counted. Shaded pixels is the percentage of all
pixels that made it to the shading stage. Blended pixels is the
percentage of shaded pixels that were alpha blended to the color buffer.
Overdraw measures how many pixels were shaded vs. how many pixels the
render target has. e.g. a 640x480 render target has 307200 pixels. If
exactly that many pixels are shaded the overdraw number will read 0%.
614400 shaded pixels will read as an overdraw of 100%.
Sampler calls is simply the number of times sampler.sample_2d() was
called.
