support uniform textures and other compose methods

[fisherevans]

This PR does two things:

• allow users to pass in uniform shaders
• add support for Screen and Multiply composition methods.

Shader example

var someTex *glhf.Texture
var canvas *opengl.Canvas
canvas.SetUniformTexture("uBackgroundTexture", someTex, 0)

This is how you would pass in the uniform texture to a shader like this

uniform sampler2D uBackgroundTexture;

Compose example

Using canvas.SetComposeMethod(pixel.ComposeMultiply) allows you to achieve this kind of lighting:
https://www.slembcke.net/blog/2DLightingTechniques#screen-space-lightmaps