Add macro guards for OpenGL <=1.3 function prototypes

[Semphriss]

Description

I've added two macros in SDL_opengl.h to help with dynamically loading OpenGL:

• #define SDL_OPENGL_1_NO_PROTOTYPES to hide function prototypes
• #define SDL_OPENGL_1_FUNCTION_TYPEDEFS to add PFNGL*PROC function types

They help avoid clashing names for OpenGL functions up to 1.3 when developers use a different or a custom OpenGL loader.

I've also added the typedefs (PFNGL...PROC) for those functions. Since they seem not to be standard, I made another macro to hide them by default. I created them with gedit 41.0 find-and-replace, with find = GLAPI ([a-zA-Z0-9 \*]+) GLAPIENTRY (gl[a-zA-Z0-9]+) *\(([^)]+)\); and replace = typedef \1 (APIENTRYP PFN\U\2\EPROC) (\3);. This is probably compatible with other regex engines; the least common-looking feature I used is \U for uppercase and \E for un-uppercase.

I've kept the two macros separate because someone might want to have both the prototypes and the typedefs.

As usual, I'm open to changing the macro names and their structure.

Existing Issue(s)

Closes #10606