Use OpenGL to get GL_MAX_TEXTURE_SIZE instead of hard-coding static values

[teridon]

The preferences dialog currently has a hard-coded value of 2048 for the maximum texture size, but it is possible for OpenGL to support larger texture sizes.

Use OpenGL functions (e.g. glGetIntegerv()) to get the GL_MAX_TEXTURE_SIZE value, and use that instead.

[mserquet]

Changed the max value in Pre-release 3.0.19. However, exports bigger than 2048x2048 with either the PFSS or HEK Layer active still have issues.