glUniform may silently error

[lcnr]

this concerns the internal api of backend/state.rs.

Once update_program errors, there might not be a valid program loaded.
As we do not check this when updating uniforms, we currently ignore potential errors.

This could be observed by calling draw after a previous call to draw has failed because of update_program. I do not know how to actually test the impact of this problem.

[lcnr]

somewhat solved by panicking if glUseProgram fails