added ofPixels::setColor(ofColor) and ofImage::setColor(ofColor)

example:

````c++
#include "ofAppGlutWindow.h"
#include "ofMain.h"

class ofApp : public ofBaseApp {
public:
	ofImage img;
	
	void setup() {
		img.allocate(ofGetWidth(), ofGetHeight(), OF_IMAGE_COLOR);
		img.update();
	}
	
	void update() {
		img.setColor(ofColor::fromHsb(mouseX % 256, mouseY % 256, 255));
		img.update();
	}
	
	void draw() {
		img.draw(0, 0);
	}
};

int main() {
	ofAppGlutWindow window;
	ofSetupOpenGL(&window, 1280, 720, OF_WINDOW);
	ofRunApp(new ofApp());
}
````