ShaderFilter doesn't seem to work with BitmapData.applyFilter()

Haxe
#1

I’ve been trying to use openfl Shaders from within HaxeFlixel, using ShaderFilter. So as a test I wrote this:

class PlayState extends FlxState
{
	var testbitmapdata:BitmapData;
	var sp:FlxSprite;
	override public function create()
	{
		testbitmapdata = Assets.getBitmapData("assets/images/test.png");
		
		var redshader = new Shader();
		redshader.glVertexSource = Assets.getText("assets/data/shaders/simpleredshader/redvertshader.vert");
		redshader.glFragmentSource = Assets.getText("assets/data/shaders/simpleredshader/redfragshader.frag");

		testbitmapdata.applyFilter(testbitmapdata, testbitmapdata.rect, new Point(), new ShaderFilter(redshader));


		sp = new FlxSprite(50, 100);
		sp.pixels = testbitmapdata;
		sp.setGraphicSize(100);
		sp.updateHitbox();
		add(sp);
	}

	override public function update(elapsed:Float)
	{
		super.update(elapsed);
	}
}

My shaders look like this:
Vertex Shader:

attribute float openfl_Alpha;
attribute vec4 openfl_ColorMultiplier;
attribute vec4 openfl_ColorOffset;
attribute vec4 openfl_Position;
attribute vec2 openfl_TextureCoord;

varying float openfl_Alphav;
varying vec4 openfl_ColorMultiplierv;
varying vec4 openfl_ColorOffsetv;
varying vec2 openfl_TextureCoordv;

uniform mat4 openfl_Matrix;
uniform bool openfl_HasColorTransform;
uniform vec2 openfl_TextureSize;

void main(void)
{
    gl_Position = openfl_Matrix * openfl_Position;
}

Fragment Shader: (for this test I just wanted to output the color red to see if the shader worked)

void main(void)
{
    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
}

But it seems like the applyFilter() method does not seem to work. The sprite does not turn red as I would expect. However, when I apply the same shader object as a filter to an FlxCamera (using sp.camera.setFilters([ new ShaderFilter(redshader) ]) ) it does turn the screen red, as expected.

Is there something else I should have added to the shader code or the haxe code to make it work for the flxsprite?