gradient shaders

[uwerat]

The current implementation of QskGradientMaterial uses shaders loaded from the quickshapes module. As the algos in those shaders are pretty simple it would make sense to implement our own set of shaders to drop the extra module dependency.

Furthermore it would be good to have specific shaders for gradients made of 2 colors - a very common use case. These shaders do not need precalculated color tables ( -> textures ) as such an interpolation ( https://registry.khronos.org/OpenGL-Refpages/gl4/html/mix.xhtml ) can be done easily in the shader code.