Alpha-blending

### Is your feature request related to a problem? Please describe.
Alpha-blending seem chellengable for this enormous support of the different types of channels.

### Describe the solution you'd like
How about start on support the same RGB views?

#### C++ Example
template
auto alpha_blend(const ViewT& view1, const ViewT& view2, ptr_diff_t x=0, ptr_diff_t y=0){
...
}

```cpp
#include
namespace gil = boost::gil;
int main
{
...
auto v3 = alpha_blend(v1, v2);
...
}
```

### Describe alternatives you've considered
I imagine using boost::compute for this.

### Additional context
https://stackoverflow.com/questions/746899/how-to-calculate-an-rgb-colour-by-specifying-an-alpha-blending-amount