feat(flame_3d): Add UnlitMaterial support (#3914)

Add `UnlitMaterial` support

Author: wolfenrain

packages/flame_3d/assets/shaders/unlit_material.shaderbundle

@@ -1,2 +1,3 @@
 export 'material/material.dart';
 export 'material/spatial_material.dart';
+export 'material/unlit_material.dart';

packages/flame_3d/lib/src/resources/material.dart

@@ -0,0 +1,52 @@
+import 'dart:ui' hide FragmentShader;
+
+import 'package:flame_3d/graphics.dart';
+import 'package:flame_3d/resources.dart';
+
+/// {@template unlit_material}
+/// A material that renders textures without any lighting calculations.
+///
+/// Unlike [SpatialMaterial], this material outputs the texture color multiplied
+/// by [albedoColor] directly, with no PBR shading, tone mapping, or gamma
+/// correction. This makes it ideal for:
+///
+/// - 2D canvas content rendered onto 3D surfaces
+/// - UI panels and HUD elements in 3D space
+/// - Emissive/self-lit surfaces
+/// - Sky boxes and debug rendering
+/// {@endtemplate}
+class UnlitMaterial extends Material {
+  /// {@macro unlit_material}
+  UnlitMaterial({
+    this.albedoColor = const Color(0xFFFFFFFF),
+    Texture? albedoTexture,
+  }) : albedoTexture = albedoTexture ?? Texture.standard,
+       super(
+         vertexShader: VertexShader.fromAsset(
+           'packages/flame_3d/assets/shaders/unlit_material.shaderbundle',
+           slots: ['VertexInfo'],
+         ),
+         fragmentShader: FragmentShader.fromAsset(
+           'packages/flame_3d/assets/shaders/unlit_material.shaderbundle',
+           slots: ['albedoTexture', 'Material'],
+         ),
+       );
+
+  /// The material's base color, multiplied with [albedoTexture].
+  Color albedoColor;
+
+  /// The texture to render. Multiplied by [albedoColor].
+  Texture albedoTexture;
+
+  @override
+  void apply(covariant RenderContext3D context) {
+    vertexShader
+      ..setMatrix4('VertexInfo.model', context.model)
+      ..setMatrix4('VertexInfo.view', context.view)
+      ..setMatrix4('VertexInfo.projection', context.projection);
+
+    fragmentShader
+      ..setTexture('albedoTexture', albedoTexture)
+      ..setColor('Material.albedoColor', albedoColor);
+  }
+}

packages/flame_3d/lib/src/resources/material/unlit_material.dart

@@ -0,0 +1,19 @@
+#version 460 core
+
+in vec2 fragTexCoord;
+in vec4 fragColor;
+in vec3 fragPosition;
+in vec3 fragNormal;
+
+out vec4 outColor;
+
+uniform sampler2D albedoTexture;
+
+uniform Material {
+  vec4 albedoColor;
+} material;
+
+void main() {
+  vec4 texColor = texture(albedoTexture, fragTexCoord);
+  outColor = texColor * material.albedoColor;
+}

packages/flame_3d/shaders/unlit_material.frag

@@ -0,0 +1,32 @@
+#version 460 core
+
+in vec3 vertexPosition;
+in vec2 vertexTexCoord;
+in vec4 vertexColor;
+in vec3 vertexNormal;
+in vec4 vertexJoints;
+in vec4 vertexWeights;
+
+out vec2 fragTexCoord;
+out vec4 fragColor;
+out vec3 fragPosition;
+out vec3 fragNormal;
+
+uniform VertexInfo {
+  mat4 model;
+  mat4 view;
+  mat4 projection;
+} vertex_info;
+
+void main() {
+  mat4 mvp = vertex_info.projection * vertex_info.view * vertex_info.model;
+  gl_Position = mvp * vec4(vertexPosition, 1.0);
+
+  fragTexCoord = vertexTexCoord;
+  fragColor = vertexColor;
+
+  // Pass through all vertex attributes so the compiler doesn't strip them,
+  // which would break the vertex buffer layout.
+  fragPosition = vertexPosition + vertexJoints.xyz * vertexWeights.x;
+  fragNormal = vertexNormal;
+}