Fix badly specified RGB10_A2 format

There is a bug in RGB10_A2 format for lwjgl implementation - it was specified as RBG instead of RGBA texture. It was causing GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT_EXT when trying to use it with MRT.

There might be same problem in other renderers (? jogl, android, ios), I do not compile these projects, so cannot check/fix there - please take a look.