Added GUI render and support for faces without textures

- Added FlatRenderProgram to render 2D graphics
  - Can be used as-is
  - Supports rectangular coordinate-aligned nested masks
- Added AssembledFlatLayer to cache 2D graphics
  - Use RenderTarget for add elements during reassembly
- ShapeRenderProgram now allows null textures (defaults to white BG)
- Sprite now deduces size based on primitive's content size
  - unless given size explicitly
- ShapeRenderHelper.*WorldTransform methods renamed to .*Transform

src/main/resources/assets/shaders/Flat.fragment.glsl

@@ -0,0 +1,13 @@
+#version 120
+
+uniform ivec2 maskStart;
+uniform ivec2 maskEnd;
+
+void applyMask() {
+	if (
+			gl_FragCoord.x < maskStart.x || gl_FragCoord.x >= maskEnd.x ||
+			gl_FragCoord.y < maskStart.y || gl_FragCoord.y >= maskEnd.y
+	) {
+		discard;
+	}
+}

src/main/resources/assets/shaders/Flat.vertex.glsl

@@ -0,0 +1,5 @@
+#version 120
+
+void flatTransferToFragment() {
+	shapeTransferToFragment();
+}

src/main/resources/assets/shaders/FlatDefault.fragment.glsl

@@ -0,0 +1,8 @@
+#version 120
+
+void main(void) {
+	applyTexture();
+	applyColorMultiplier();
+	applyAlpha();
+	applyMask();
+}

src/main/resources/assets/shaders/FlatDefault.vertex.glsl

@@ -0,0 +1,6 @@
+#version 120
+
+void main(void) {
+	gl_Position = applyFinalTransform(vec4(inputPositions, 1.0));
+	flatTransferToFragment();
+}

src/main/resources/assets/shaders/Shape.fragment.glsl

@@ -7,14 +7,20 @@ uniform sampler2D textureSlot;
 uniform vec2 textureStart;
 uniform vec2 textureSize;
 
+uniform bool useTexture;
+
 void applyTexture() {
-	gl_FragColor = texture2D(
-			textureSlot,
-			vec2(
-				varyingTextureCoords[0] * textureSize[0] + textureStart[0],
-				varyingTextureCoords[1] * textureSize[1] + textureStart[1]
-			)
-	);
+	if (!useTexture) {
+		gl_FragColor = vec4(1, 1, 1, 1);
+	} else {
+		gl_FragColor = texture2D(
+				textureSlot,
+				vec2(
+					varyingTextureCoords[0] * textureSize[0] + textureStart[0],
+					varyingTextureCoords[1] * textureSize[1] + textureStart[1]
+				)
+		);
+	}
 }
 
 void multiply(inout vec4 vector, float scalar) {