fix sdl2 keymap on non-apple systems and add rpi doc

doc/Developer-QuickStart-Rpi.txt

@@ -0,0 +1,17 @@
+# When building on Raspberry Pi, you need to get the hardware
+# accelerated graphics driver enabled with SDL2.  
+# We borrow the SDL2 build from RetroPie which takes care of this for us.
+
+git clone https://github.com/RetroPie/RetroPie-Setup
+cd RetroPie-Setup
+sudo ./retropie_packages.sh sdl2
+
+
+# Then build GSplus like any other *nix system
+sudo apt-get update -y
+sudo apt-get install -y libpcap0.8-dev libsdl2-image-dev
+git clone https://github.com/digarok/gsplus.git
+cd gsplus/src
+ln -s vars_rpilinux_sdl2 vars
+make
+

src/sdl2_driver.c

@@ -181,8 +181,13 @@ int a2_key_to_sdlkeycode[][3] = {
   { 0x54,       SDLK_KP_2, SDLK_DOWN },
   { 0x55,       SDLK_KP_3, SDLK_PAGEDOWN },
   { 0x36,       SDLK_RCTRL, SDLK_LCTRL },
+  #if defined(__APPLE__)
   { 0x3a,       SDLK_LALT, SDLK_RALT },         /* Option */
   { 0x37,       SDLK_LGUI, SDLK_RGUI },         /* Command */
+  #else
+  { 0x3a,       SDLK_LGUI, SDLK_RGUI },         /* Command */
+  { 0x37,       SDLK_LALT, SDLK_RALT },         /* Option */
+  #endif
   { 0x31,       ' ', 0 },
   { 0x3b,       SDLK_LEFT, 0 },
   { 0x3d,       SDLK_DOWN, 0 },