Make iced_core::Button customizable

Now it supports:
  - Any kind of content
  - Custom border radius
  - Custom background

wgpu/src/shader/quad.frag

@@ -3,6 +3,7 @@
 layout(location = 0) in vec4 v_Color;
 layout(location = 1) in vec2 v_Pos;
 layout(location = 2) in vec2 v_Scale;
+layout(location = 3) in flat uint v_BorderRadius;
 
 layout(location = 0) out vec4 o_Color;
 
@@ -26,8 +27,11 @@ float rounded(in vec2 frag_coord, in vec2 position, in vec2 size, float radius,
 }
 
 void main() {
-    o_Color = vec4(
-        v_Color.xyz,
-        v_Color.w * rounded(gl_FragCoord.xy, v_Pos, v_Scale, 5.0, 1.0)
-    );
+    float radius_alpha = 1.0;
+
+    if(v_BorderRadius > 0.0) {
+        radius_alpha = rounded(gl_FragCoord.xy, v_Pos, v_Scale, v_BorderRadius, 1.0);
+    }
+
+    o_Color = vec4(v_Color.xyz, v_Color.w * radius_alpha);
 }

wgpu/src/shader/quad.vert

@@ -4,6 +4,7 @@ layout(location = 0) in vec2 v_Pos;
 layout(location = 1) in vec2 i_Pos;
 layout(location = 2) in vec2 i_Scale;
 layout(location = 3) in vec4 i_Color;
+layout(location = 4) in uint i_BorderRadius;
 
 layout (set = 0, binding = 0) uniform Globals {
     mat4 u_Transform;
@@ -12,6 +13,7 @@ layout (set = 0, binding = 0) uniform Globals {
 layout(location = 0) out vec4 o_Color;
 layout(location = 1) out vec2 o_Pos;
 layout(location = 2) out vec2 o_Scale;
+layout(location = 3) out uint o_BorderRadius;
 
 void main() {
     mat4 i_Transform = mat4(
@@ -24,6 +26,7 @@ void main() {
     o_Color = i_Color;
     o_Pos = i_Pos;
     o_Scale = i_Scale;
+    o_BorderRadius = i_BorderRadius;
 
     gl_Position = u_Transform * i_Transform * vec4(v_Pos, 0.0, 1.0);
 }