Set `egui::RawInput::max_texture_side`

[jakoschiko]

According to the documentation of egui::RawInput::max_texture_side this field needs to be set based on information from the graphics driver. Since egui-miniquad manages RawInput I think it's responsible of doing that.

I noticed it because I was not able to load an image larger than 2048 pixels with egui::Context::load_texture. The function checks the size and panics. I'm not aware of any workaround.