feat(disp): Enable rendering to display subsection (#2583)

This change introduces new fields on `lv_disp_drv_t` that allow to
specify the size of the full display and the offset of the display
subsection that is being rendered to. The values are used to transform
the drawing area before calling `flush_cb` so that only the desired part
of the full display is being rendered to.

Relates to: lvgl/lv_drivers#166

docs/CHANGELOG.md

@@ -40,6 +40,7 @@
 - feat(btnmatrix/keyboard): add option to show popovers on button press
 - fix(types) LV_FORMAT_ATTRIBUTE now works with gnu version greater than 4.4
 - feat(event) add add LV_EVENT_CHILD_CREATED/DELETED
+- feat(disp): Enable rendering to display subsection
 - feat(keyboard): add user-defined modes
 
 ## v8.0.2 (16.07.2021)

docs/porting/display.md

@@ -69,6 +69,10 @@ LVGL might render the screen in multiple chunks and therefore call `flush_cb` mu
 
 ### Optional fields 
 There are some optional display driver data fields:
+- `physical_hor_res` horizontal resolution of the full / physical display in pixels. Only set this when _not_ using the full screen (defaults to -1 / same as `hor_res`).
+- `physical_ver_res` vertical resolution of the full / physical display in pixels. Only set this when _not_ using the full screen (defaults to -1 / same as `ver_res`).
+- `offset_x` horizontal offset from the the full / physical display in pixels. Only set this when _not_ using the full screen (defaults to 0).
+- `offset_y` vertical offset from the the full / physical display in pixels. Only set this when _not_ using the full screen (defaults to 0).
 - `color_chroma_key` A color which will be drawn as transparent on chrome keyed images. Set to `LV_COLOR_CHROMA_KEY` from `lv_conf.h` by default.
 - `anti_aliasing` use anti-aliasing (edge smoothing). Enabled by default if `LV_COLOR_DEPTH` is set to at least 16 in `lv_conf.h`.
 - `rotated` and `sw_rotate` See the [Rotation](#rotation) section below.