main: Added entry points for NamedRenderbufferStorage/Multisample

main: Added entry points for NamedRenderbufferStorage/Multisample

v2: Review from Laura Ekstrand
- get rid of a change that should not have happened in this patch
- improve the error messages
- fix alignments
- fix a capitalization in a function name in an error message

v3: Review from Laura Ekstrand
- move the test for the validity of the renderbuffer to less generic
  functions
- get rid of some changes that accidentally landed in the wrong commit
- revert some alignment fixes

v3: Review from Laura Ekstrand
- check that the lookup returns a valid renderbuffer
- cosmetic changes to some error messages

Reviewed-by: Laura Ekstrand <laura@jlekstrand.net>
Signed-off-by: Martin Peres <martin.peres@linux.intel.com>