Fixed text vanishing when the transform is vertically flipped

src/nanovg.c

@@ -2454,6 +2454,12 @@ static void nvg__renderText(NVGcontext* ctx, NVGvertex* verts, int nverts)
 	ctx->textTriCount += nverts/3;
 }
 
+static int   nvg__isTransformFlipped( const float *xform)
+{
+	float det = xform[0] * xform[3] - xform[2] * xform[1];
+	return( det < 0);
+}
+
 float nvgText(NVGcontext* ctx, float x, float y, const char* string, const char* end)
 {
 	NVGstate* state = nvg__getState(ctx);
@@ -2464,6 +2470,8 @@ float nvgText(NVGcontext* ctx, float x, float y, const char* string, const char*
 	float invscale = 1.0f / scale;
 	int cverts = 0;
 	int nverts = 0;
+	int isFlipped;
+	float tmp;
 
 	if (end == NULL)
 		end = string + strlen(string);
@@ -2480,6 +2488,7 @@ float nvgText(NVGcontext* ctx, float x, float y, const char* string, const char*
 	verts = nvg__allocTempVerts(ctx, cverts);
 	if (verts == NULL) return x;
 
+	isFlipped = nvg__isTransformFlipped( state->xform);
 	fonsTextIterInit(ctx->fs, &iter, x*scale, y*scale, string, end, FONS_GLYPH_BITMAP_REQUIRED);
 	prevIter = iter;
 	while (fonsTextIterNext(ctx->fs, &iter, &q)) {
@@ -2497,6 +2506,11 @@ float nvgText(NVGcontext* ctx, float x, float y, const char* string, const char*
 				break;
 		}
 		prevIter = iter;
+		if( isFlipped)
+		{
+			tmp = q.y0; q.y0 = q.y1; q.y1 = tmp;
+			tmp = q.t0; q.t0 = q.t1; q.t1 = tmp;
+		}
 		// Transform corners.
 		nvgTransformPoint(&c[0],&c[1], state->xform, q.x0*invscale, q.y0*invscale);
 		nvgTransformPoint(&c[2],&c[3], state->xform, q.x1*invscale, q.y0*invscale);