Upload only changed rows to GPU and hoist glyph bounds checks

paintIncremental's per-row dirty work was negated by present() calling
PixelBuffer.updateBuffer(null), which re-uploads the whole pane texture
every frame. Track the vertical band of buffer rows written since the
last present and hand that to updateBuffer so only changed rows upload.
The border is now drawn without extending the dirty band (its pixels are
unchanged between incremental frames). Also clamp blitGlyph's rectangle
once instead of bounds-checking every glyph pixel in the inner loop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.settings/org.eclipse.buildship.core.prefs

@@ -5,7 +5,7 @@ connection.gradle.distribution=GRADLE_DISTRIBUTION(LOCAL_INSTALLATION(/home/anon
 connection.project.dir=
 eclipse.preferences.version=1
 gradle.user.home=
-java.home=/nix/store/c3pl7bqrx3d2rc3dh98z6yaj0mv1p52g-openjdk-21.0.10+7/lib/openjdk
+java.home=/home/anon/.local/lib/graalvm-jdk-21.0.9+7.1
 jvm.arguments=
 offline.mode=false
 override.workspace.settings=true