doc: Document coordinate system expectations for borrowed textures

[tronical]

No description provided.

[tronical]

Verified using the following patch that renders the thumbs up image the correct way by default:

diff --git a/examples/opengl_texture/Cargo.toml b/examples/opengl_texture/Cargo.toml
index 4a12dc04d..d12a6e34f 100644
--- a/examples/opengl_texture/Cargo.toml
+++ b/examples/opengl_texture/Cargo.toml
@@ -18,6 +18,7 @@ name = "opengl_texture"
 slint = { path = "../../api/rs/slint" }
 glow = { version = "0.12" }
 instant = { version = "0.1", features = [ "now" ] }
+image = { version = "0.24.0" }
 
 [build-dependencies]
 slint-build = { path = "../../api/rs/build" }
diff --git a/examples/opengl_texture/main.rs b/examples/opengl_texture/main.rs
index f5582e458..eee744825 100644
--- a/examples/opengl_texture/main.rs
+++ b/examples/opengl_texture/main.rs
@@ -154,6 +154,7 @@ struct DemoRenderer {
     start_time: instant::Instant,
     displayed_texture: DemoTexture,
     next_texture: DemoTexture,
+    image: image::RgbaImage,
 }
 
 impl DemoRenderer {
@@ -308,6 +309,11 @@ fn new(gl: glow::Context) -> Self {
             let displayed_texture = DemoTexture::new(&gl, 320, 200);
             let next_texture = DemoTexture::new(&gl, 320, 200);
 
+            let image =
+                image::open(concat!(env!("CARGO_MANIFEST_DIR"), "/../gallery/thumbsup.png"))
+                    .unwrap()
+                    .into_rgba8();
+
             Self {
                 gl,
                 program,
@@ -318,6 +324,7 @@ fn new(gl: glow::Context) -> Self {
                 start_time: instant::Instant::now(),
                 displayed_texture,
                 next_texture,
+                image,
             }
         }
     }
@@ -355,6 +362,66 @@ fn render(
                 std::mem::swap(&mut self.next_texture, &mut new_texture);
             }
 
+            let target_image = image::imageops::resize(
+                &self.image,
+                self.next_texture.width,
+                self.next_texture.height,
+                image::imageops::FilterType::Nearest,
+            );
+
+            unsafe {
+                let _saved_texture_binding =
+                    ScopedTextureBinding::new(gl, Some(self.next_texture.texture));
+
+                let old_unpack_alignment = gl.get_parameter_i32(glow::UNPACK_ALIGNMENT);
+                let old_unpack_row_length = gl.get_parameter_i32(glow::UNPACK_ROW_LENGTH);
+                let old_unpack_skip_pixels = gl.get_parameter_i32(glow::UNPACK_SKIP_PIXELS);
+                let old_unpack_skip_rows = gl.get_parameter_i32(glow::UNPACK_SKIP_ROWS);
+
+                gl.pixel_store_i32(glow::UNPACK_ALIGNMENT, 1);
+                gl.tex_parameter_i32(
+                    glow::TEXTURE_2D,
+                    glow::TEXTURE_MIN_FILTER,
+                    glow::LINEAR as i32,
+                );
+                gl.tex_parameter_i32(
+                    glow::TEXTURE_2D,
+                    glow::TEXTURE_MAG_FILTER,
+                    glow::LINEAR as i32,
+                );
+                gl.tex_parameter_i32(
+                    glow::TEXTURE_2D,
+                    glow::TEXTURE_WRAP_S,
+                    glow::CLAMP_TO_EDGE as i32,
+                );
+                gl.tex_parameter_i32(
+                    glow::TEXTURE_2D,
+                    glow::TEXTURE_WRAP_T,
+                    glow::CLAMP_TO_EDGE as i32,
+                );
+                gl.pixel_store_i32(glow::UNPACK_ROW_LENGTH, width as i32);
+                gl.pixel_store_i32(glow::UNPACK_SKIP_PIXELS, 0);
+                gl.pixel_store_i32(glow::UNPACK_SKIP_ROWS, 0);
+
+                gl.tex_image_2d(
+                    glow::TEXTURE_2D,
+                    0,
+                    glow::RGBA as _,
+                    target_image.width() as _,
+                    target_image.height() as _,
+                    0,
+                    glow::RGBA,
+                    glow::UNSIGNED_BYTE,
+                    Some(target_image.as_raw().align_to().1),
+                );
+
+                gl.pixel_store_i32(glow::UNPACK_ALIGNMENT, old_unpack_alignment);
+                gl.pixel_store_i32(glow::UNPACK_ROW_LENGTH, old_unpack_row_length);
+                gl.pixel_store_i32(glow::UNPACK_SKIP_PIXELS, old_unpack_skip_pixels);
+                gl.pixel_store_i32(glow::UNPACK_SKIP_ROWS, old_unpack_skip_rows);
+            }
+
+            /*
             self.next_texture.with_texture_as_active_fbo(|| {
                 let mut saved_viewport: [i32; 4] = [0, 0, 0, 0];
                 gl.get_parameter_i32_slice(glow::VIEWPORT, &mut saved_viewport);
@@ -379,6 +446,7 @@ fn render(
                     saved_viewport[3],
                 );
             });
+            */
 
             gl.use_program(None);
         }