shl: shader: add "len" parameter to shader sources

shl: shader: add "len" parameter to shader sources

Shader sources may not be 0 terminated if we mmap() them. Hence, we need
to pass the length to the shader compiler. As glShaderSource() allows this
<0 as zero-terminated strings, we can simply add a parameter for the
shader length.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>