GPUFramebuffer: Bump max attachment count to 8

This is needed by eevee-rewrite

source/blender/gpu/intern/gpu_framebuffer_private.hh

@@ -43,8 +43,9 @@ typedef enum GPUAttachmentType : int {
   GPU_FB_COLOR_ATTACHMENT3,
   GPU_FB_COLOR_ATTACHMENT4,
   GPU_FB_COLOR_ATTACHMENT5,
-  /* Number of maximum output slots.
-   * We support 6 outputs for now (usually we wouldn't need more to preserve fill rate). */
+  GPU_FB_COLOR_ATTACHMENT6,
+  GPU_FB_COLOR_ATTACHMENT7,
+  /* Number of maximum output slots. */
   /* Keep in mind that GL max is GL_MAX_DRAW_BUFFERS and is at least 8, corresponding to
    * the maximum number of COLOR attachments specified by glDrawBuffers. */
   GPU_FB_MAX_ATTACHMENT,

source/blender/gpu/opengl/gl_framebuffer.hh

@@ -141,6 +141,8 @@ static inline GLenum to_gl(const GPUAttachmentType type)
     ATTACHMENT(COLOR_ATTACHMENT3);
     ATTACHMENT(COLOR_ATTACHMENT4);
     ATTACHMENT(COLOR_ATTACHMENT5);
+    ATTACHMENT(COLOR_ATTACHMENT6);
+    ATTACHMENT(COLOR_ATTACHMENT7);
     default:
       BLI_assert(0);
       return GL_COLOR_ATTACHMENT0;