Add FBO support for GLES

To support FBO for iOS, the current framebuffer and renderbuffer must be restored after calling nvgluCreateFramebuffer(). The framebuffer should also be restored when calling nvgluBindFramebuffer(NULL) as well.

src/nanovg_gl_utils.h

@@ -35,7 +35,7 @@ void nvgluDeleteFramebuffer(NVGcontext* ctx, NVGLUframebuffer* fb);
 
 #ifdef NANOVG_GL_IMPLEMENTATION
 
-#if defined(NANOVG_GL3)
+#if defined(NANOVG_GL3) || defined(NANOVG_GLES2) || defined(NANOVG_GLES3)
 // FBO is core in OpenGL 3>.
 #	define NANOVG_FBO_VALID 1
 #elif defined(NANOVG_GL2)
@@ -46,14 +46,21 @@ void nvgluDeleteFramebuffer(NVGcontext* ctx, NVGLUframebuffer* fb);
 #	endif
 #endif
 
+static GLint defaultFBO = -1;
+
 NVGLUframebuffer* nvgluCreateFramebuffer(NVGcontext* ctx, int w, int h, int imageFlags)
 {
 #ifdef NANOVG_FBO_VALID
+	GLint defaultFBO;
+	GLint defaultRBO;
 	NVGLUframebuffer* fb = NULL;
 	fb = (NVGLUframebuffer*)malloc(sizeof(NVGLUframebuffer));
 	if (fb == NULL) goto error;
 	memset(fb, 0, sizeof(NVGLUframebuffer));
 
+	glGetIntegerv(GL_FRAMEBUFFER_BINDING, &defaultFBO);
+	glGetIntegerv(GL_RENDERBUFFER_BINDING, &defaultRBO);
+
 	fb->image = nvgCreateImageRGBA(ctx, w, h, imageFlags | NVG_IMAGE_FLIPY | NVG_IMAGE_PREMULTIPLIED, NULL);
 	fb->texture = nvglImageHandle(ctx, fb->image);
 
@@ -72,6 +79,8 @@ NVGLUframebuffer* nvgluCreateFramebuffer(NVGcontext* ctx, int w, int h, int imag
 
 	if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE) goto error;
 
+	glBindFramebuffer(GL_FRAMEBUFFER, defaultFBO);
+	glBindRenderbuffer(GL_RENDERBUFFER, defaultRBO);
 	return fb;
 error:
 	nvgluDeleteFramebuffer(ctx, fb);
@@ -88,7 +97,8 @@ error:
 void nvgluBindFramebuffer(NVGLUframebuffer* fb)
 {
 #ifdef NANOVG_FBO_VALID
-	glBindFramebuffer(GL_FRAMEBUFFER, fb != NULL ? fb->fbo : 0);
+	if (defaultFBO == -1) glGetIntegerv(GL_FRAMEBUFFER_BINDING, &defaultFBO);
+	glBindFramebuffer(GL_FRAMEBUFFER, fb != NULL ? fb->fbo : defaultFBO);
 #else
 	NVG_NOTUSED(fb);
 #endif