Improve framebuffer handling on Zephyr

[dani-clo]

Respect the native display dimensions when not rotated.

Clear all LTDC framebuffers during initialization.

Add getDrawBuffer() to access a non-displayed RGB565 framebuffer when available, without exposing LTDC-specific details.

This allows users to render directly into the framebuffer and display the result without additional copies:

auto *dst = static_cast<uint16_t *>(Display.getDrawBuffer());
// Render directly into the destination buffer
Display.drawBuffer(0, 0, dst);

[github-actions]

Memory usage change @ 3f712e3

Board	flash	%	RAM for global variables	%
arduino:zephyr_main:giga	🔺 +124 - +208	+0.01 - +0.02	🔺 +56 - +88	+0.02 - +0.03
arduino:zephyr_main:portentah7	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board	examples/ArduinoLogo flash	%	examples/ArduinoLogo RAM for global variables	%	examples/ArduinoLogoDrawing flash	%	examples/ArduinoLogoDrawing RAM for global variables	%	examples/LVGLDemo flash	%	examples/LVGLDemo RAM for global variables	%
arduino:zephyr_main:giga	124	0.01	56	0.02	124	0.01	56	0.02	208	0.02	88	0.03
arduino:zephyr_main:portentah7	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0

Click for full report CSV

Board,examples/ArduinoLogo<br>flash,%,examples/ArduinoLogo<br>RAM for global variables,%,examples/ArduinoLogoDrawing<br>flash,%,examples/ArduinoLogoDrawing<br>RAM for global variables,%,examples/LVGLDemo<br>flash,%,examples/LVGLDemo<br>RAM for global variables,%
arduino:zephyr_main:giga,124,0.01,56,0.02,124,0.01,56,0.02,208,0.02,88,0.03
arduino:zephyr_main:portentah7,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0