Fixed bug 3690 - SDL2 KMS/DRM render context support

Manuel

The attached patch adds support for KMS/DRM context graphics.

It builds with no problem on X86_64 GNU/Linux systems, provided the needed libraries are present, and on ARM GNU/Linux systems that have KMS/DRM support and a GLES2 implementation.
Tested on Raspberry Pi: KMS/DRM is what the Raspberry Pi will use as default in the near future, once the propietary DispmanX API by Broadcom is overtaken by open graphics stack, it's possible to boot current Raspbian system in KMS mode by adding "dtoverlay=vc4-kms-v3d" to config.txt on Raspbian's boot partition.
X86 systems use KMS right away in every current GNU/Linux system.

Simple build instructions:

$./autogen.sh
$./configure --enable-video-kmsdrm
$make

CMakeLists.txt

@@ -314,6 +314,8 @@ set_option(VIDEO_COCOA         "Use Cocoa video driver" ${APPLE})
 set_option(DIRECTX             "Use DirectX for Windows audio/video" ${WINDOWS})
 set_option(RENDER_D3D          "Enable the Direct3D render driver" ${WINDOWS})
 set_option(VIDEO_VIVANTE       "Use Vivante EGL video driver" ${UNIX_SYS})
+set_option(VIDEO_KMSDRM        "Use KMS DRM video driver" ${UNIX_SYS})
+dep_option(KMSDRM_SHARED       "Dynamically load KMS DRM support" ON "VIDEO_KMSDRM" OFF)
 
 # TODO: We should (should we?) respect cmake's ${BUILD_SHARED_LIBS} flag here
 # The options below are for compatibility to configure's default behaviour.
@@ -917,6 +919,7 @@ elseif(UNIX AND NOT APPLE AND NOT ANDROID)
     CheckOpenGLESX11()
     CheckWayland()
     CheckVivante()
+    CheckKMSDRM()
   endif()
 
   if(LINUX)