Attempt to negate the need to repetitively call reload() on context-switch

The current implementation forces to call `reload()` each time a context is bound. This is, as far as I know, due to the wgl-Specification stating that the function-adresses depend upon the context (or it's pixel format).
In this pull request I offer a first attempt for a solution to this.

Instead of loading the opengl function pointers into global variables as `reload()` does, the new function `reloadOnce(out functions)` takes an object named `functions` of type `ContextDependendFunctions` as a parameter. This object has function-pointer members for all opengl-functions that depend on the context (in other words that are not loaded by `loadSymbols()`). The `reloadOnce()` function fills these members with the appropriate function pointers for the currently bound context. 

Instead of calling `reload()` each time a context is bound, you can simply call `reloadOnce()` for each context directly after the creation of the context and store the resulting `functions` object along with the context. Instead of calling opengl functions globally, you can now call the functions using the pointers in the `functions` object. When a new context is bound, there is no more need to call `reload()`, but instead you must use the corresponding `functions` object for this context.

Note that is is a first attempt and criticism is welcome. Also I preserved the original `reload()` functionality for downward compatibility. Also ARB extensions are not yet transfered in to the new system.

Example
```d
// For core API functions.
import derelict.opengl3.gl3;

void main() {
    // Load OpenGL versions 1.0 and 1.1.
    DerelictGL3.load();

    // Create an OpenGL context with another library (like SDL 2 or GLFW 3)
    ...

    // For each context do once:
    // Load versions 1.2+
    ContextDependentFunctions functions;
    DerelictGL3.reload(functions);

    // Now OpenGL functions can be called.
    functions.gl...(...);
    ...
}
```