LVGL fix memory allocation of flush buffers

[s-hadinger]

Description:

Fix memory allocation since LVGL 9:

• memory buffers were allocated 50% bigger than necessary, and considered by LVGL as twice as small as they were
• allocate flush buffers preferably in internal memory, instead of PSRAM (applies especially for ESP32S3)
• fix typo in fluslines

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.6
• The code change is tested and works with Tasmota core ESP32 V.3.0.0
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass