feat(ui): add Rect widget with fill, color, and parent-relative sizing

assets/shaders/rect_f_shader.glsl

@@ -0,0 +1,9 @@
+#version 460
+
+out vec4 color;
+
+uniform vec4 inColor;
+
+void main(void) {
+    color = inColor;
+}

assets/shaders/rect_v_shader.glsl

@@ -0,0 +1,11 @@
+#version 460
+
+layout (location = 0) in vec2 pos;
+
+
+uniform mat4 model_matrix;
+uniform mat4 proj_matrix;
+
+void main(void) {
+    gl_Position = proj_matrix * model_matrix * vec4(pos, 0.0, 1.0);
+}