metal renderer: use the device address space instead of the constant address space for colors.

It doesn't negatively impact GPU performance in my tests, and it removes the need for 256 byte memory alignment of color data on macOS.

src/render/metal/SDL_render_metal.m

@@ -962,7 +962,7 @@ static int
 METAL_QueueSetDrawColor(SDL_Renderer *renderer, SDL_RenderCommand *cmd)
 {
     const size_t vertlen = sizeof (float) * 4;
-    float *verts = (float *) SDL_AllocateRenderVertices(renderer, vertlen, CONSTANT_ALIGN, &cmd->data.color.first);
+    float *verts = (float *) SDL_AllocateRenderVertices(renderer, vertlen, 16, &cmd->data.color.first);
     if (!verts) {
         return -1;
     }

src/render/metal/SDL_shaders_metal.metal

@@ -20,7 +20,7 @@ vertex SolidVertexOutput SDL_Solid_vertex(const device float2 *position [[buffer
     return v;
 }
 
-fragment float4 SDL_Solid_fragment(constant float4 &col [[buffer(0)]])
+fragment float4 SDL_Solid_fragment(const device float4 &col [[buffer(0)]])
 {
     return col;
 }
@@ -44,7 +44,7 @@ vertex CopyVertexOutput SDL_Copy_vertex(const device float2 *position [[buffer(0
 }
 
 fragment float4 SDL_Copy_fragment(CopyVertexOutput vert [[stage_in]],
-                                  constant float4 &col [[buffer(0)]],
+                                  const device float4 &col [[buffer(0)]],
                                   texture2d<float> tex [[texture(0)]],
                                   sampler s [[sampler(0)]])
 {
@@ -60,7 +60,7 @@ struct YUVDecode
 };
 
 fragment float4 SDL_YUV_fragment(CopyVertexOutput vert [[stage_in]],
-                                 constant float4 &col [[buffer(0)]],
+                                 const device float4 &col [[buffer(0)]],
                                  constant YUVDecode &decode [[buffer(1)]],
                                  texture2d<float> texY [[texture(0)]],
                                  texture2d_array<float> texUV [[texture(1)]],
@@ -77,7 +77,7 @@ fragment float4 SDL_YUV_fragment(CopyVertexOutput vert [[stage_in]],
 }
 
 fragment float4 SDL_NV12_fragment(CopyVertexOutput vert [[stage_in]],
-                                 constant float4 &col [[buffer(0)]],
+                                 const device float4 &col [[buffer(0)]],
                                  constant YUVDecode &decode [[buffer(1)]],
                                  texture2d<float> texY [[texture(0)]],
                                  texture2d<float> texUV [[texture(1)]],
@@ -93,7 +93,7 @@ fragment float4 SDL_NV12_fragment(CopyVertexOutput vert [[stage_in]],
 }
 
 fragment float4 SDL_NV21_fragment(CopyVertexOutput vert [[stage_in]],
-                                 constant float4 &col [[buffer(0)]],
+                                 const device float4 &col [[buffer(0)]],
                                  constant YUVDecode &decode [[buffer(1)]],
                                  texture2d<float> texY [[texture(0)]],
                                  texture2d<float> texUV [[texture(1)]],