EGL: Fix creation of single-buffered windows

The EGL backend ignored the state of GLFW_DOUBLEBUFFER and always
created a double-buffered EGL window.

This sets the EGL_RENDER_BUFFER attribute at EGL window creation
appropriately.

Fixes #1843.

src/egl_context.c

@@ -173,7 +173,7 @@ static GLFWbool chooseEGLConfig(const _GLFWctxconfig* ctxconfig,
         u->stencilBits = getEGLConfigAttrib(n, EGL_STENCIL_SIZE);
 
         u->samples = getEGLConfigAttrib(n, EGL_SAMPLES);
-        u->doublebuffer = GLFW_TRUE;
+        u->doublebuffer = desired->doublebuffer;
 
         u->handle = (uintptr_t) n;
         usableCount++;
@@ -643,6 +643,9 @@ GLFWbool _glfwCreateContextEGL(_GLFWwindow* window,
             setAttrib(EGL_GL_COLORSPACE_KHR, EGL_GL_COLORSPACE_SRGB_KHR);
     }
 
+    if (!fbconfig->doublebuffer)
+        setAttrib(EGL_RENDER_BUFFER, EGL_SINGLE_BUFFER);
+
     setAttrib(EGL_NONE, EGL_NONE);
 
     native = _glfwPlatformGetEGLNativeWindow(window);

src/egl_context.h

@@ -64,6 +64,8 @@
 #define EGL_OPENGL_ES_API 0x30a0
 #define EGL_OPENGL_API 0x30a2
 #define EGL_NONE 0x3038
+#define EGL_RENDER_BUFFER 0x3086
+#define EGL_SINGLE_BUFFER 0x3085
 #define EGL_EXTENSIONS 0x3055
 #define EGL_CONTEXT_CLIENT_VERSION 0x3098
 #define EGL_NATIVE_VISUAL_ID 0x302e