back-port of texture-padding feature from GLES2 branch

[tom-braun]

in order to get rid of bleeding textures when using the BlackPawnTextureBuilder I back-ported the padding code from the GLES2 branch. The changes are straight forward. i just copied the corresponding code from the GLES2 branch to master. I took care to neither brake coding-style nor compatibility with existing code. I tested the code by introducing it to my app. This required absolutely no changes to my app's code. To make use of the padding feature however, one needs to use the new constructor of BlackPawnTextureBuilder providing it a border-spacing and padding value.

[nicolasgramlich]

Not mergable. Also are you trying to backport a feature from GLES2 to master? (Even though you are filing this PR against GLES2 and not master.)