Set source texture unit.

Outputs/OpenGL/CompositionShader.cpp

@@ -336,7 +336,8 @@ OpenGL::Shader OpenGL::composition_shader(
 	const int source_width,
 	const int source_height,
 	const int target_width,
-	const int target_height
+	const int target_height,
+	const GLenum source_texture_unit
 ) {
 	//
 	// Compose and compiler shader.
@@ -416,6 +417,7 @@ OpenGL::Shader OpenGL::composition_shader(
 	shader.set_uniform("cyclesSinceRetraceMultiplier", cycles_multiplier);
 	shader.set_uniform("sourceSize", float(source_width), float(source_height));
 	shader.set_uniform("targetSize", float(target_width), float(target_height));
+	shader.set_uniform("source", GLint(source_texture_unit - GL_TEXTURE0));
 
 	return shader;
 }

Outputs/OpenGL/CompositionShader.hpp

@@ -48,7 +48,8 @@ Shader composition_shader(
 	int source_width,
 	int source_height,
 	int target_width,
-	int target_height
+	int target_height,
+	GLenum source_texture_unit
 );
 
 }

Outputs/OpenGL/ScanTarget.cpp

@@ -141,7 +141,8 @@ ScanTarget::ScanTarget(const API api, const GLuint target_framebuffer, const flo
 			pair.second,
 			sample_multiplier,
 			2048, 2048,
-			buffer_width, 2048
+			buffer_width, 2048,
+			GL_TEXTURE0
 		).bind();
 	}
 }