arch merge lv_demos

demos/benchmark/README.md

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+# Benchmark demo
+
+## Overview
+
+The benchmark demo tests the performance in various cases. 
+For example rectangle, border, shadow, text, image blending, image transformation, bleding modes, etc. 
+All tests are repeated with 50% opacity.
+
+The size an position of the objects during testing are set with a pseudo random number to make the benchmark repeatable. 
+
+On to top of the screen the title of the current test step, and the result of the previous step is displayed.
+
+## Run the benchmark
+- In `lv_ex_conf.h` set `LV_USE_DEMO_BENCHMARK 1`
+- After `lv_init()` and initializing the drivers call `lv_demo_benchmark()`
+
+## Interpret the result
+
+The FPS is measured like this:
+- load the next step
+- in the display driver's `monitor_cb` accumulate the time-to-render and the number of cycles 
+- measure for 1 second
+- calculate `FPS = time_sum / render_cnt`
+
+Note that it can result in very high FPS results for simple cases. 
+E.g. if some simple rectangles are drawn in 5 ms, the benchmark will tell it's 200 FPS. 
+So it ignores `LV_DISP_REFR_PERIOD` which tells LVGL how often it should refresh the screen. 
+In other words, the benchmark shows the FPS from the pure rendering time.
+
+By default, only the changed areas are refreshed. It means if only a few pixels are changed in 1 ms the benchmark will show 1000 FPS. To measure the performance with full screen refresh uncomment `lv_obj_invalidate(lv_scr_act())` in `monitor_cb()` in `lv_demo_benchmark.c`.
+
+![LVGL benchmark running](https://github.com/lvgl/lv_examples/blob/master/src/lv_demo_benchmark/screenshot1.png?raw=true)
+
+
+## Result summary 
+In the end, a table is created to display measured FPS values.
+
+On top of the summary screen, the "Weighted FPS" value is shown. 
+In this, the result of the more common cases are taken into account with a higher weight. 
+
+"Opa. speed" shows the speed of the measurements with opacity compared to full opacity. 
+E.g. "Opa. speed = 90%" means that rendering with opacity is 10% slower. 
+
+In the first section of the table, "Slow but common cases", those cases are displayed which are considered common but were slower than 20 FPS.
+
+Below this in the "All cases section" all the results are shown. The < 10 FPS results are shown with red, the >= 10 but < 20 FPS values are displayed with orange.
+
+
+![LVGL benchmark result summary](https://github.com/lvgl/lv_examples/blob/master/src/lv_demo_benchmark/screenshot2.png?raw=true)