internal/graphicscommand: reland: add graphics.MaxVertexFloatCount and use it instead of IndicesCount

This change is reland of 54e2790a06
but with a fix for WebGL. Also, this changes the logic when len(vertices)
exceeds the maximum: just remove the last part.

Updates #2460

internal/graphics/vertex.go

@@ -14,6 +14,10 @@
 
 package graphics
 
+import (
+	"math"
+)
+
 const (
 	ShaderImageCount = 4
 
@@ -48,8 +52,18 @@ const (
 )
 
 const (
-	IndicesCount     = (1 << 16) / 3 * 3 // Adjust num for triangles. TODO: Remove this (#2460).
+	IndicesCount = (1 << 16) / 3 * 3 // Adjust num for triangles. TODO: Remove this (#2460).
+)
+
+const (
 	VertexFloatCount = 8
+
+	// MaxVertexCount is the maximum number of vertices for one draw call.
+	// This value is 2^16 - 1 = 65535, as the index type is uint16.
+	// This value cannot be exactly 2^16 == 65536 especially with WebGL 2, as 65536th vertex is not rendered correctly.
+	// See https://registry.khronos.org/webgl/specs/latest/2.0/#5.18 .
+	MaxVertexCount      = math.MaxUint16
+	MaxVertexFloatCount = MaxVertexCount * VertexFloatCount
 )
 
 var (

internal/graphicscommand/command.go

@@ -86,11 +86,16 @@ func (q *commandQueue) appendIndices(indices []uint16, offset uint16) {
 
 // mustUseDifferentVertexBuffer reports whether a different vertex buffer must be used.
 func mustUseDifferentVertexBuffer(nextNumVertexFloats int) bool {
-	return nextNumVertexFloats > graphics.IndicesCount*graphics.VertexFloatCount
+	return nextNumVertexFloats > graphics.MaxVertexFloatCount
 }
 
 // EnqueueDrawTrianglesCommand enqueues a drawing-image command.
 func (q *commandQueue) EnqueueDrawTrianglesCommand(dst *Image, srcs [graphics.ShaderImageCount]*Image, offsets [graphics.ShaderImageCount - 1][2]float32, vertices []float32, indices []uint16, blend graphicsdriver.Blend, dstRegion, srcRegion graphicsdriver.Region, shader *Shader, uniforms []uint32, evenOdd bool) {
+	if len(vertices) > graphics.MaxVertexFloatCount {
+		// The last part cannot be specified by indices. Just omit them.
+		vertices = vertices[:graphics.MaxVertexFloatCount]
+	}
+
 	split := false
 	if mustUseDifferentVertexBuffer(q.tmpNumVertexFloats + len(vertices)) {
 		q.tmpNumVertexFloats = 0