Merge pull request #121 from sgraham/zero-w-scissor

use -1 as the 'no scissor' value, rather than 0, so that 0 width still scissors

src/nanovg.c

@@ -519,8 +519,8 @@ void nvgReset(struct NVGcontext* ctx)
 	state->alpha = 1.0f;
 	nvgTransformIdentity(state->xform);
 
-	state->scissor.extent[0] = 0.0f;
-	state->scissor.extent[1] = 0.0f;
+	state->scissor.extent[0] = -1.0f;
+	state->scissor.extent[1] = -1.0f;
 
 	state->fontSize = 16.0f;
 	state->letterSpacing = 0.0f;
@@ -815,6 +815,9 @@ void nvgScissor(struct NVGcontext* ctx, float x, float y, float w, float h)
 {
 	struct NVGstate* state = nvg__getState(ctx);
 
+	w = nvg__maxf(0.0f, w);
+	h = nvg__maxf(0.0f, h);
+
 	nvgTransformIdentity(state->scissor.xform);
 	state->scissor.xform[4] = x+w*0.5f;
 	state->scissor.xform[5] = y+h*0.5f;
@@ -828,8 +831,8 @@ void nvgResetScissor(struct NVGcontext* ctx)
 {
 	struct NVGstate* state = nvg__getState(ctx);
 	memset(state->scissor.xform, 0, sizeof(state->scissor.xform));
-	state->scissor.extent[0] = 0;
-	state->scissor.extent[1] = 0;
+	state->scissor.extent[0] = -1.0f;
+	state->scissor.extent[1] = -1.0f;
 }
 
 static int nvg__ptEquals(float x1, float y1, float x2, float y2, float tol)

src/nanovg_gl.h

@@ -750,7 +750,7 @@ static int glnvg__convertPaint(struct GLNVGcontext* gl, struct GLNVGfragUniforms
 	frag->innerCol = glnvg__premulColor(paint->innerColor);
 	frag->outerCol = glnvg__premulColor(paint->outerColor);
 
-	if (scissor->extent[0] < 0.5f || scissor->extent[1] < 0.5f) {
+	if (scissor->extent[0] < -0.5f || scissor->extent[1] < -0.5f) {
 		memset(frag->scissorMat, 0, sizeof(frag->scissorMat));
 		frag->scissorExt[0] = 1.0f;
 		frag->scissorExt[1] = 1.0f;