Fixed BGRA channel assignment for Color struct

[thomascp]

Hi,

First of all, thank you so much for this amazing project! It has been incredibly helpful for learning Raspberry Pi bare-metal GPU programming.

While running the code, I noticed that my rendered triangle looked different from the one shown in your README.
In my case, the bottom-left vertex is blue, the bottom-right is green, and the top vertex is red.

After some investigation, I found that StoreTileBufferGeneral stores the buffer using the V3D_OUTPUT_IMAGE_FORMAT_RGBA8 format. However, when it is copied over to the framebuffer, the framebuffer expects a BGRA format.

To fix this color channel mismatch, I modified the codes in drawTex (swapping the r and b struct members) so that the memory layout aligns correctly with the expected BGRA format.

struct Color
{
...
    union
    {
        struct
        {
            u8 b; // Swapped to match BGRA
            u8 g;
            u8 r; // Swapped to match BGRA
            u8 a;
        };
        u32 argb;
    };
...
};

Could you please take a look? Thank you!

BR,
Peng