GLSL: Improve copy_to_dev and copy_to_host
GLSL: Improve copy_to_dev and copy_to_host

- don't use GL_UNPACK_ROW_LENGTH, which is unavailable in OpenGL ES
- only transfer directly to/from host buffer if memory is contiguous
- honor min[] values of associated buffer


Former-commit-id: 5a6d2ba3fba2ae17472d697bc05347df7cf27949
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide