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.

(cherry picked from commit 114776a24605418e6d719d2f30141e351e93c6e0)

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++;
@@ -596,6 +596,9 @@ 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);
 
     window->context.egl.surface =

src/egl_context.h

@@ -80,6 +80,8 @@ typedef struct wl_egl_window* EGLNativeWindowType;
 #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