backend/gl: fix rendering when corner_radius is 0

Fixes #1311

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/gl/shaders.c

@@ -154,24 +154,26 @@ const char blit_shader_glsl[] = GLSL(330,
 			border_color.rgb = border_color.rgb * (max_brightness / brightness);
 		}
 
-		// Rim color is the color of the outer rim of the window, if there is no
-		// border, it's the color of the window itself, otherwise it's the border.
-		// Using mix() to avoid a branch here.
-		vec4 rim_color = mix(c, border_color, clamp(border_width, 0.0f, 1.0f));
+		if (corner_radius != 0) {
+			// Rim color is the color of the outer rim of the window, if there is no
+			// border, it's the color of the window itself, otherwise it's the border.
+			// Using mix() to avoid a branch here.
+			vec4 rim_color = mix(c, border_color, clamp(border_width, 0.0f, 1.0f));
 
-		vec2 outer_size = effective_size;
-		vec2 inner_size = outer_size - vec2(corner_radius) * 2.0f;
-		// +1.0 so the last 1-pixel ring of the rounded rectangle will transition
-		// smoothly from 1 to 0 for anti-aliasing. If we don't do this, everything
-		// inside the corner radius will be solid, and we will have an extra 1-pixel
-		// feathering outside the corner radius, which makes it look bad.
-		float rect_distance = rectangle_sdf(texcoord - outer_size / 2.0f,
-		    inner_size / 2.0f) - corner_radius + 1.0f;
-		if (rect_distance > 0.0f) {
-			c = (1.0f - clamp(rect_distance, 0.0f, 1.0f)) * rim_color;
-		} else {
-			float factor = clamp(rect_distance + border_width, 0.0f, 1.0f);
-			c = (1.0f - factor) * c + factor * border_color;
+			vec2 outer_size = effective_size;
+			vec2 inner_size = outer_size - vec2(corner_radius) * 2.0f;
+			// +1.0 so the last 1-pixel ring of the rounded rectangle will transition
+			// smoothly from 1 to 0 for anti-aliasing. If we don't do this, everything
+			// inside the corner radius will be solid, and we will have an extra 1-pixel
+			// feathering outside the corner radius, which makes it look bad.
+			float rect_distance = rectangle_sdf(texcoord - outer_size / 2.0f,
+			    inner_size / 2.0f) - corner_radius + 1.0f;
+			if (rect_distance > 0.0f) {
+				c = (1.0f - clamp(rect_distance, 0.0f, 1.0f)) * rim_color;
+			} else {
+				float factor = clamp(rect_distance + border_width, 0.0f, 1.0f);
+				c = (1.0f - factor) * c + factor * border_color;
+			}
 		}
 
 		return c;