Route Canvas erase setup through helper

src/legacy_canvas_stroke_erase_services.h

@@ -11,12 +11,14 @@ struct LegacyStrokeEraseUniforms {
     int texture_slot = 0;
     int stroke_texture_slot = 1;
     int mask_texture_slot = 2;
+    float alpha = 1.0f;
     bool mask_enabled = false;
 };
 
 struct LegacyStrokeEraseShaderExecution {
     std::function<void(kShader)> use_shader;
     std::function<void(kShaderUniform, int)> set_int;
+    std::function<void(kShaderUniform, float)> set_float;
     std::function<void(kShaderUniform, const glm::mat4&)> set_mat4;
 };
 
@@ -25,6 +27,7 @@ struct LegacyStrokeEraseShaderExecution {
     return {
         .use_shader = [](kShader shader) { ShaderManager::use(shader); },
         .set_int = [](kShaderUniform uniform, int value) { ShaderManager::u_int(uniform, value); },
+        .set_float = [](kShaderUniform uniform, float value) { ShaderManager::u_float(uniform, value); },
         .set_mat4 = [](kShaderUniform uniform, const glm::mat4& value) { ShaderManager::u_mat4(uniform, value); },
     };
 }
@@ -38,6 +41,7 @@ inline void setup_legacy_stroke_erase_shader(
     execution.set_int(kShaderUniform::TexStroke, uniforms.stroke_texture_slot);
     execution.set_int(kShaderUniform::TexMask, uniforms.mask_texture_slot);
     execution.set_int(kShaderUniform::Mask, uniforms.mask_enabled);
+    execution.set_float(kShaderUniform::Alpha, uniforms.alpha);
     execution.set_mat4(kShaderUniform::MVP, uniforms.mvp);
 }