Renamed struct gl_context to struct __GLcontextRec.
Include glcore.h, setup GL imports/exports.
Replaced gl_ prefix with _mesa_ prefix in context.[ch] functions.
GLcontext's Visual field is no longer a pointer.
ctx->Driver.Index/ColorMask() now return void.
Removed SWmasking and SWLogicOpEnabled variables.
LogicOps and color/index masking are no longer special-case device
driver functions. The Xlib driver was the only driver that used
them. Things are more uniform now.