Improved framerate control code - strip.show(), strip.service()

[softhack007]

The old code was having lots of minor errors in calculation of "time passed" since last frame. Additionally, the value for MIN_SHOW_DELAY (_frametime < 16 ? 8 : 15) seems very random and it's effectively limiting the max framerates to 60-80fps.

This change implements a better solution:

• Separates framerate control in strip.service() from FPS calculation in strip.show()
• framerate calcutation considers all steps of rendering (drawing + brightness limiter + busses.show). The old code was only using the time after last "show" until beginning of next render cycle - so it artificially created too low framerates and random "stuttering" of effects.
• Add an "unlimited" mode where all effects will render as fast as possible (limited to 333 fps to keep WiFi alive)

From user perspective:

• smoothly running effects at all framerates
• The "Target FPS" (LED settings) will be achieved (and not exceeded much) if effects can render fast enough.
• setting Target FPS to 250 means "unlimited" (good for testing render performance)
• -D WLEDMM_FASTPATH is needed for the new code
• 8266 and non-fastpath builds are using the improved code, however with the old MIN_SHOW_DELAY that causes too many idle cycles