Prevents uploading unused vertices to GPU.

This commit fixes the bug that `glnvg__renderFill()` allocates unused quad vertices for `GLNVG_CONVEXFILL`, which causes unused vertices uploaded to the GPU.

Besides, the quad for the `GLNVG_FILL` type is changed to drawn by `GL_TRIANGLE_STRIP` instead of `GL_TRIANGLES` so two more vertices can be reduced per quad.

In one of my test case, I need to paste an image to a framebuffer about 5,000 times, and this change would reduce the total number of uploaded vertices from `52,106` to `22,476`. It is good for FPS and battery consumption.

src/nanovg_gl.h

@@ -1028,7 +1028,7 @@ static void glnvg__fill(GLNVGcontext* gl, GLNVGcall* call)
 	// Draw fill
 	glnvg__stencilFunc(gl, GL_NOTEQUAL, 0x0, 0xff);
 	glStencilOp(GL_ZERO, GL_ZERO, GL_ZERO);
-	glDrawArrays(GL_TRIANGLES, call->triangleOffset, call->triangleCount);
+	glDrawArrays(GL_TRIANGLE_STRIP, call->triangleOffset, call->triangleCount);
 
 	glDisable(GL_STENCIL_TEST);
 }
@@ -1346,6 +1346,7 @@ static void glnvg__renderFill(void* uptr, NVGpaint* paint, NVGcompositeOperation
 	if (call == NULL) return;
 
 	call->type = GLNVG_FILL;
+	call->triangleCount = 4;
 	call->pathOffset = glnvg__allocPaths(gl, npaths);
 	if (call->pathOffset == -1) goto error;
 	call->pathCount = npaths;
@@ -1353,10 +1354,13 @@ static void glnvg__renderFill(void* uptr, NVGpaint* paint, NVGcompositeOperation
 	call->blendFunc = glnvg__blendCompositeOperation(compositeOperation);
 
 	if (npaths == 1 && paths[0].convex)
+	{
 		call->type = GLNVG_CONVEXFILL;
+		call->triangleCount = 0;	// Bounding box fill quad not needed for convex fill
+	}
 
 	// Allocate vertices for all the paths.
-	maxverts = glnvg__maxVertCount(paths, npaths) + 6;
+	maxverts = glnvg__maxVertCount(paths, npaths) + call->triangleCount;
 	offset = glnvg__allocVerts(gl, maxverts);
 	if (offset == -1) goto error;
 
@@ -1378,20 +1382,16 @@ static void glnvg__renderFill(void* uptr, NVGpaint* paint, NVGcompositeOperation
 		}
 	}
 
-	// Quad
-	call->triangleOffset = offset;
-	call->triangleCount = 6;
-	quad = &gl->verts[call->triangleOffset];
-	glnvg__vset(&quad[0], bounds[0], bounds[3], 0.5f, 1.0f);
-	glnvg__vset(&quad[1], bounds[2], bounds[3], 0.5f, 1.0f);
-	glnvg__vset(&quad[2], bounds[2], bounds[1], 0.5f, 1.0f);
-
-	glnvg__vset(&quad[3], bounds[0], bounds[3], 0.5f, 1.0f);
-	glnvg__vset(&quad[4], bounds[2], bounds[1], 0.5f, 1.0f);
-	glnvg__vset(&quad[5], bounds[0], bounds[1], 0.5f, 1.0f);
-
 	// Setup uniforms for draw calls
 	if (call->type == GLNVG_FILL) {
+		// Quad
+		call->triangleOffset = offset;
+		quad = &gl->verts[call->triangleOffset];
+		glnvg__vset(&quad[0], bounds[2], bounds[3], 0.5f, 1.0f);
+		glnvg__vset(&quad[1], bounds[2], bounds[1], 0.5f, 1.0f);
+		glnvg__vset(&quad[2], bounds[0], bounds[3], 0.5f, 1.0f);
+		glnvg__vset(&quad[3], bounds[0], bounds[1], 0.5f, 1.0f);
+
 		call->uniformOffset = glnvg__allocFragUniforms(gl, 2);
 		if (call->uniformOffset == -1) goto error;
 		// Simple shader for stencil