Add integer attributes to Geometry

[gooostaw]

Until now, you could send integer data to the vertex shader, but it was converted to float. I used vertexAttribIPointer to make it easier to use integer attributes.

[gordonnl]

Sorry it took so long to get to this - could you please give a brief explanation of the blendFunc changes? Is it primarily to support the OES_draw_buffers_indexed extension?
Thanks a lot for your work!

[gooostaw]

Indeed, this pull request does more than just add integer attributes. Only commit 0f7da7d should be included in this PR, sorry. The other commits add support for OES_draw_buffers_indexed. Not sure if anyone will need this. This allows you to set a different blending for each draw buffer. To use this feature, you need to pass an array of blendFunc objects instead of a single object to the blendFunc attribute of the Program class. Each element in the array corresponds to a specific draw buffer. While the usage is simple, the implementation required some changes.