LV_LAYER_SIMPLE_BUF_SIZE macro

"Simple layers" are used when a widget has `style_opa < 255` to buffer the widget into a layer and blend it as an image with the given opacity. Note that `bg_opa`, `text_opa` etc don't require buffering into layer) The widget can be buffered in smaller chunks to avoid using large buffers. - LV_LAYER_SIMPLE_BUF_SIZE: [bytes] the optimal target buffer size. LVGL will try to allocate it - LV_LAYER_SIMPLE_FALLBACK_BUF_SIZE: [bytes] used if `LV_LAYER_SIMPLE_BUF_SIZE` couldn't be allocated. Both buffer sizes are in bytes. "Transformed layers" (where transform_angle/zoom properties are used) use larger buffers and can't be drawn in chunks. So these settings affects only widgets with opacity.