Fractional GUI scale (0.5 step), fit menu to window, fix settings pages

- display.gui-scale is now a number: 0 (auto) or 0.5..4 in 0.5 steps
- explicit scale is no longer limited by window size (tiled windows);
  only an open menu page is shrunk to fit the window
- snap Batch2D vertices to pixels on fractional scale (fixes dots above text)
- pass app to settings_common via options (not visible inside modules)
- gui.get_scale() returns a number

src/graphics/core/Batch2D.cpp

@@ -44,6 +44,10 @@ void Batch2D::vertex(
     float u, float v,
     float r, float g, float b, float a
 ) {
+    if (pixelSnap > 0.0f) {
+        x = std::round(x * pixelSnap) / pixelSnap;
+        y = std::round(y * pixelSnap) / pixelSnap;
+    }
     buffer[index].position = {x, y};
     buffer[index].uv = {u * region.getWidth() + region.u1, v * region.getHeight() + region.v1};
     buffer[index].color = {r, g, b, a};
@@ -54,6 +58,9 @@ void Batch2D::vertex(
     glm::vec2 uvpoint,
     float r, float g, float b, float a
 ) {
+    if (pixelSnap > 0.0f) {
+        point = glm::round(point * pixelSnap) / pixelSnap;
+    }
     buffer[index].position = point;
     buffer[index].uv = {uvpoint.x * region.getWidth() + region.u1, uvpoint.y * region.getHeight() + region.v1};
     buffer[index].color = {r, g, b, a};
@@ -83,6 +90,10 @@ void Batch2D::setRegion(UVRegion region) {
     this->region = region;
 }
 
+void Batch2D::setPixelSnap(float pixelsPerUnit) {
+    pixelSnap = pixelsPerUnit;
+}
+
 void Batch2D::point(float x, float y, float r, float g, float b, float a){
     if (index + 6 >= capacity)
         flush();