Support channel blending

The GUI can blend the left and other channel together using the blending parameter. This allows depth or right channels to be mixed with the colour to see edge quality etc.

applications/gui/src/screen.cpp

@@ -48,11 +48,13 @@ namespace {
 
 	constexpr char const *const defaultImageViewFragmentShader =
 		R"(#version 330
-		uniform sampler2D image;
+		uniform sampler2D image1;
+		uniform sampler2D image2;
+		uniform float blendAmount;
 		out vec4 color;
 		in vec2 uv;
 		void main() {
-			color = texture(image, uv);
+			color = blendAmount * texture(image1, uv) + (1.0 - blendAmount) * texture(image2, uv);
 		})";
 }
 
@@ -457,8 +459,12 @@ void ftl::gui::Screen::draw(NVGcontext *ctx) {
 					size().x() * r, size().y() * r);*/
 			mShader.bind();
 			glActiveTexture(GL_TEXTURE0);
-			glBindTexture(GL_TEXTURE_2D, mImageID);
-			mShader.setUniform("image", 0);
+			glBindTexture(GL_TEXTURE_2D, leftEye_);
+			glActiveTexture(GL_TEXTURE1);
+			glBindTexture(GL_TEXTURE_2D, (camera_->getRight().isValid()) ? rightEye_ : leftEye_);
+			mShader.setUniform("image1", 0);
+			mShader.setUniform("image2", 1);
+			mShader.setUniform("blendAmount", (camera_->getChannel() != ftl::codecs::Channel::Left) ? root_->value("blending", 0.5f) : 1.0f);
 			mShader.setUniform("scaleFactor", scaleFactor);
 			mShader.setUniform("position", imagePosition);
 			mShader.drawIndexed(GL_TRIANGLES, 0, 2);