Android build and crash fixes

Fixes `load_shader` failing on Android due to an unrelated error flag. Also silences some compiler warnings.
I think what we should do here is still check the error, but make sure that get_error() is cleared before we try to compile a shader. You can do this by calling get_error() in a loop until it returns NO_ERROR (see http://www.khronos.org/opengles/sdk/docs/man/xhtml/glGetError.xml for details).