fix: remove incorrect alpha_blend in bgimage.slang

old-shaders/rounded_rect.vert.glsl

@@ -0,0 +1,22 @@
+#version 140
+
+
+
+#line 0 7893001
+#define left 0
+#define top 1
+#define right 2
+#define bottom 3
+
+const ivec2 vertex_pos_map[4] = ivec2[4](
+    ivec2(right, top),
+    ivec2(right, bottom),
+    ivec2(left, bottom),
+    ivec2(left, top)
+);
+const vec4 dest_rect = vec4(-1, 1, 1, -1);
+
+void main() {
+    ivec2 pos = vertex_pos_map[gl_VertexID];
+    gl_Position = vec4(dest_rect[pos.x], dest_rect[pos.y], 0, 1);
+}