player: use the same constness as gles2 implementation for shader sources
player: use the same constness as gles2 implementation for shader sources

This is fixing compilation warnings/errors of discarded const qualifieds
(gcc -Wdiscarded-qualifiers).

Signed-off-by: Julien Olivain <7fdfe229fce69a4d7f38653f3755ccbae5703f88@cotds.org>

jolivain/glslsandbox-player,jolivain/glslsandbox-player,jolivain/glslsandbox-player,jolivain/glslsandbox-player