Fix movement stopping when activating saucer after longjump

In hindsight my emscripten-ifdef (70d416a8f63f) is nonsense. As fpcrtl_glShaderSource() would not be defined and lead to compiling issues. So either it's 3 ifdefs (in pas2cRedo, pas2cSystem and misc.c), in order to toggle between fpcrtl_ and the native function, or alternatively have no ifdef for it at all. I'm going with none at all, which means emscripten will compile with the original (const) function prototype, being wrapped by the fpcrtl_ function, same as non-emscripten builds.

Rewrite a few CMake flag descriptions

fix shader uniform access

fix bug #750: workaround GL2 function params const-iness issues with pas2c

fix condition

transpose atlas sprites where necessary

add function to retrieve texture data

fix buffer sizes

fix gear lookup allocated on stack

ship the atlas to the gpu

add an empty texture constructor