Fixes circles not rendering on Mesa 24.1.

Fixes: maplibre#4062
See discussion here: maplibre#4402 (reply in thread)

src/shaders/circle.fragment.glsl

@@ -19,11 +19,9 @@ void main() {
     #pragma mapbox: initialize lowp float stroke_opacity
 
     vec2 extrude = v_data.xy;
+    float antialiased_blur = v_data.z;
     float extrude_length = length(extrude);
 
-    lowp float antialiasblur = v_data.z;
-    float antialiased_blur = -max(blur, antialiasblur);
-
     float opacity_t = smoothstep(0.0, antialiased_blur, extrude_length - 1.0);
 
     float color_t = stroke_width < 0.01 ? 0.0 : smoothstep(antialiased_blur, 0.0, extrude_length - radius / (radius + stroke_width));

src/shaders/circle.vertex.glsl

@@ -62,7 +62,7 @@ void main(void) {
     // This is a minimum blur distance that serves as a faux-antialiasing for
     // the circle. since blur is a ratio of the circle's size and the intent is
     // to keep the blur at roughly 1px, the two are inversely related.
-    lowp float antialiasblur = 1.0 / u_device_pixel_ratio / (radius + stroke_width);
+    lowp float antialiasblur = -max(1.0 / u_device_pixel_ratio / radius, blur);
 
     v_data = vec3(extrude.x, extrude.y, antialiasblur);
 }